By Taraka Mantra we basically mean the Pranavam, the first sound or OM, also called the Nada Brahman.
So, Taraka: Deliverer.
Taraka Mantra: From the root word tara–that which crosses. The Taraka Mantra is that which enables
its invokers to cross over the ocean of samsara and attain liberation.
Taraka Nama: The Delivering Name; Om
From Sannyasa Upanishad:
That which is Om is the indestructible, the supreme and Brahman. That
alone should be worshipped. It is this that is of the eight subtle
syllables. And this becomes eight, being of eight forms. ‘A’ is the
first letter; ‘U’ is the second; ‘M’ is the third; Bindu is the
fourth; Nada is the fifth; Kala is the sixth; Kalatita (that beyond
Kala) is the seventh; and that which is beyond these is the eighth.
It is called Taraka, because it enables one to cross this mundane existence. Know that Taraka alone is Brahman and it alone should be
- When confronted with a mass of sins (i.e. when the flesh becomes weak, over-ruling the dictates of wisdom) he shall practise the
(meaningful) repetition of the Taraka (Om) twelve thousand times (a
day); for it cuts (sins).
Now, it's mentioned in the Vedas themselves (viz in Sri Rudram) that Lord Rudra liberates.
Namas taraya cha (prostrations to one who liberates)
And, it's also mentioned that Lord Shiva (or Rudra) is but Om in the Mantric level.
"atha kasmAduchyata o~NkAro yasmAduchchAryamANa eva
prANAnUrdhvamutkrAmayati tasmAduchyate o~NkAraH "
Rudra is called the personification of Pranva because he sends the
souls towards heaven, at the time of death (dissolution)
AtharvaSiras Upanishad 3.07
atha kasmAduchyate praNavaH yasmAduchchAryamANa eva
R^igyajuHsAmAtharvA~Ngirasa.n brahma brAhmaNebhyaH praNAmayati
nAmayati cha tasmAduchyate praNavaH||
He is called the “one who has the shape of Pranava“, because the
Brahmins read and propagate the Rik, Yajur, Sama and Atharva Vedas
only after reading 'OM
AtharvaSiras Upanishad 3.08
"atha kasmAduchyate tAra.n yasmAduchchAramANa eva
garbhajanmavyAdhijarAmaraNasa.nsAramahAbhayAttArayati trAyate cha
tasmAduchyate tAram.h "
He is called “Tara (Taraka - Liberator)” because he protects one from
the fear of life which consists of the fear of staying in the womb,
fear at time of birth, fear from diseases, fear from old age and fear
Quoted the above verses from this page which basically discusses the same question as you are doing.
Same thing is stated in Puranas like Linga and Shiva.
“Ishwara [the Lord] said: O dear sons, recite this Om mantra which is
identical with Me. Your knowledge shall be stabilized.” (Shiva Purana,
Vidyeshwara Samhita 10:32, 33)
So, when Lord Shiva himself is the liberator and his Mantric representation is Om or the Taraka why would he need any other Mantra or names?
Also, in Kashi Lord Shiva liberates the dead. This is bodiless liberation as opposed to Jivanmukti, where one attains liberation while living. And, the following passage states that bodiless liberation is attained by OM.
Brahma asked: "Lord, what sort of a thing is the Brahman- OM?" 13
Narayana replied: "The Brahman-OM consists of sixteen morae [NpU 208].
It is within the range of the four quadruple states, since while awake
there exist the four states beginning with the state of waking, while
dreaming there exist the four states beginning with dreaming, in deep
sleep there exist the four states beginning with deep sleep, and in
the fourth there exist the four states beginning with the fourth [NpU
190 - 191, n. 84].  In the state of waking there exist the four
forms of Visva: Visva-Visva, Visva-Taijasa, Visva-Prajfia, and
Visva-Fourth.14 In the state of dream there exist the four forms of
Taijasa: Taijasa-Visva, Taijasa-Taijasa, Taijasa-Prajfia, and
Taijasa-Fourth. In the state of deep sleep there exist the four forms
of Prajna: Prajna-Visva, Prajfia-Taijasa, Prajna-Prajna, and
Prajna-Fourth. In the fourth state there exist the four forms of the
fourth: Fourth-Visva, Fourth-Taijasa, Fourth-Prajfia, and
Fourth-Fourth. These reside respectively in the sixteen morae: the
waking-Visva in the phoneme a, the waking-Taijasa in the phoneme u,
the waking-Prajna in the phoneme m, the waking- Fourth in the
half-rnora, the dreaming-Visva in bindu, the dreaming-Taijasa in nada,
the dreaming-Prajna in kala, the dreaming-Fourth in kaldtita, the deep
sleep-Visva in sdnti, the deep sleep-Taijasa in sdntyatita, the deep
sleep-Prajna in unmam, the deep sleep-Fourth in manonmam, the
Fourth-Vi^va in pun, the Fourth-Taijasa in madhyamd, the
Fourth-Prajfia in pasyanti, and the Fourth-Fourth in para [NpU 208, n.
107].  The four morae of the waking state are portions of the
phoneme a. The four morae of the dreaming state are portions of the
pho- neme u. The four morae of the state of deep sleep are portions of
the phoneme m. The four morae of the fourth state are portions of
the half-mora. This indeed is the Brahman-OM. It should be worshipped
by Paramahamsas, Turiyatitas, and Avadhutas. Through it alone Brahman
is manifested. By means of it one attains bodiless liberation."
Paramhamsa ParivrAjaka Upanishad.
So, Taraka is indeed the OM.
Lord Brahma told them: “Sudharshana (the holy wheel of Lord Vishnu) is
that great Chakra. On its middle is written, the Taraka mantra (OM)
and also the single letter of Narasimha (Kshroum), on its six petals
of the Sudarshana six letters (Sahasrara Hum Phat) are written, on
its eight petals the eight letters (Om Namo Narayanaya).
Nrisimha Purva Tapaniya Upanishad.
Then from where did this idea crop up that Rama Nama is also Taraka?
The page linked above provides the following answer:
However I wouldn't dismiss the claims of Vaishnava friends as unreal.
Since their claim is based on Rama Uttara tapini Upanishad verses
where it is said that lord Shiva did chanting of Rama's name in kashi
where becoming pleased with him Rama appeared and gave a boon saying
whosoever gets the "shadakshari" (six lettered mantra) from Shiva (in
ears) while dying would get liberated.. Vaishnava friends need to
grasp the Tatwam behind that story instead of reading it as it is.
P.R. Ramachander who translated the Rama Tapini Upanishad has
incorrectly mentioned the Shadakshari (Six letterred mantra) as "Ram
Ramaya Namah". The sanskrit version of the upanishad doesn't mention
this mantra at all. The upanishad mentions the following thing about
the Shadakshara which is TARAKA.
"atha hainaM bhaaradvaajaH paprachchha yaaj~navalkya.n ki.n taaraka.n
ki.n taarayatiiti " (Ramatapini Upanishad-Poorvopanishat 6.2).
"Sage Bharadwaja enquired Yajnyavalkya - 'What is Taraka? What is the
one which liberates?"
"akaaraH prathamaaksharo bhavati ukaarodvitiiyaaksharo bhavati.
makaarastR^itiiyaaksharo bhavati. ardhamaatrashchaturthaaksharo
bhavati binduH pa~nchamaaksharo bhavati naadaH shhashhThaaksharo
bhavati . taarakatvaattaarako bhavati. tadeva taarakaM brahma tva.n
viddhi . tadevopaasitavyamiti j~neyam.h .
tasmaaduchyate shhaDakshara.n taarakamiti . . ya etattaarakaM brahma
braahmaNo nityamadhiite . sa paapmaana.n tarati . sa mR^ityu.n tarati
. sa brahmahatyaa.n tarati . sa bhruuNahatyaa.n tarati. sa sa.nsaara.n
tarati . sa sarva.n tarati . so.avimuktamaashrito bhavati . sa
mahaanbhavati . so.amR^itatva.n cha gachchhati " (Ramatapini
"That which has 'A' as the first syllable, 'U' as the second syllable,
'M' the third syllable, half-note is the fourth syllable, dot is the
fifth syllable, and sound is the sixth syllable; that is called the
Taraka. That when practiced liberates one from the cycles of births
and deaths. This is verily the ' Shadakshara (Six letered)' Taraka
mantra namely Omkara. That person who regularly utters this Mantra, he
gets ferried from sins, he gets ferried from death, he gets absolved
of the sins like Brahmahatya (killing of brahmana), killing of foetus,
he gets ferried from the ocean of Samsaara, he gest liberated from
everything, he becomes one with the Avimukta (Lord Shiva as the
redeemer), he becomes great, he becomes immortal".
Hence it is clear that the Six-Lettered Mantra what Rama mentioned is
One can check the Sanskrit verses of Rama Tapaniya Upanishad from here. One can also check the wrong translation given by P.R.Ramachander from here.
One reference, however, i got of Rama Brahma Taraka is from another minor Upanishad called Rama Rahasya:
The great sages led by Sanaka asked Hanuman again, “Please tell us the
mantra for meditating (or worshipping) the Taraka Brahma Rama.”
1. That king of mantra-s is the Ekakshara (one-syllabled) which is the store house of fire, which is Lord Vishnu who is lying down and which
is Lord Shiva who adorned with the crescent moon.
2. Its sage is Brahma, meter is Gayatri, Deity is Lord Rama, its anga is waxing half-Moon and its mind is the soul of fire,
Rama Rahasya Upanishad.
Do scriptures mention any other names or mantras which can be used as
Taraka mantra besides name of Rama and OM?
Yes, some other minor Upanishad mentions the Mantra "Om Namo Narayanaya" also as the Taraka.
Then Bharadvaja asked Yajnavalkya: “What is Taraka ? What is that
which causes one to cross (this mundane existence)”. To which
Yajnavalkya replied: “Om Namo Narayanaya is the Taraka.
Tara Sara Upanishad
Now, i have direct references from the Shiva Purana as regards which Mantra Lord Shiva whispers in the ears of the dead at Kashi in order to liberate them:
To understand the meaning of Pranava is to understand me. Pranava is the seed of all the lores.
It shall be understood as very subtle but possessed of great meaning even as the seed of the Banyan tree though very small contain»
a huge tree. It is the initial mantra and the essence of the Vedas.
Particularly it has me for its form.
- I am the the lord beyond attributes, the omniscenet and the omnipotent. I am Siva pervading ail but stationed in the
single-syllabled mantra Om.
They say that whatever object there is whether synthesized into one or analysed in pieces is thc meaning of Pranava, thanks to the combination and importance of the attributes.
It is the imperishable Brahman, the means of attaining all objects. Siva creates universe at the outset saying "0m".
- Since there is not much difference bctwecn thc sense and the sound 0m, this is explained thus. Siva is Pranava or Pranava is Siva.
- The brahminical sages, the scholars who realize the identity between the sense and the sound know me as the single-syllabled 0m.
- Hence he who aspires for salvation and is free from aberration shall understand Pranava the cause of all and me as the Nirguna
- O goddess, I shall give this crest-jewel of mantras at Käsi for the liberation of all Individual souls.
Lord Shiva's words to Goddess Parvati in Shiva Purana's 2nd Chapter of Kailasha Samhita (Translated by J.L.Shastri)
NOTE: The credit of finding these verses actually goes to The Destroyer :). I was unaware of them totally.