This is a tricky question because this mantra is also Vedic as well as Puranic. But a little careful analysis makes it easy to understand.
The mantra is panchAkshari mantra and not shaDAkshari i.e five syllabled mantra. Hence there is no "om" in the beginning of the mantra. While chanting the vedas, this is not chanted with omkaara.
Om is usually added to mantras to make it more powerful and sacred. It is not a mandatory practice though.
This mantra appears in the Krishna Yajurveda in 8th anuvAka of the famous "chamakam" section.
नमः साेमाय च रुद्राय च नमस्ताम्राय चारुणाय च नमः शड़्गाय च पशुपतये च नम उग्राय च भीमाय च नमाे अग्रेवधाय च दूरेवधाय च नमाे हन्त्रे च हनीयसे च नमाे वृक्ष्यभ्यो हरिकेशेभ्याे नमस्ताराय नमः शंभवे च मयाेभवे च नमः शंकराय च मयस्कराय च नमः शिवाय च शिवतराय च ॥ ८ -१॥
In this prayer there are many words which are also having "namah".
If we see its viniyoga, the rishis here is "aghora".
अस्य श्री रुद्राध्याय प्रश्न महामन्त्रस्य, अघोर ऋषिः, अनुष्टुप् चन्दः, सङ्कर्षण मूर्ति स्वरूपो योஉसावादित्यः परमपुरुषः स एष रुद्रो देवता । नमः शिवायेति बीजम् । शिवतरायेति शक्तिः । महादेवायेति कीलकम् । श्री साम्ब सदाशिव प्रसाद सिद्ध्यर्थे जपे विनियोगः ॥
This mantra also also in the Shukla Yajurveda Samhita.
Here the rishi is ParameshThi.
Further, this mantra is also available in purAnas/agamas where,
As per Linga puranam,
Guhyaadguhyataram saakshaanmoksha Jnaanamanuttamam,
Asya Mantrasya vakshyaami Rishicchandodhi Daivatam/
Beejam Shaktih Swaram varnam sthaanam chaivaaksharam prati,
Vamadevo naama Rishih Panktiscchanda udaatdhrutah/
Devataa Shiva yevaaham Mantrasyaasya varaananey,
Here, the Rishi is Sri vAmadeva and chhandas is pankti.
So, technically the mantra is given to different people at different times. It also appears in Vedas as well as puranas and tantra. It has different rishis when it comes to agama and puranas.
Hence in that way, it is NOT ONLY a Vedic mantra but also puranic mantra.
Further, it also appears in many stotras such as:
नागेन्द्रहाराय त्रिलोचनाय भस्मांगरागाय महेश्वराय।
नित्याय शुद्धाय दिगम्बराय तस्मै ‘न’काराय नम: शिवाय।।
मन्दाकिनी सलिलचन्दनचर्चिताय नन्दीश्वर प्रमथनाथ महेश्वराय।
मन्दारपुष्पबहुपुष्प सुपूजिताय तस्मै ‘म’काराय नम: शिवाय। (shiva panchAkshara stotram)
Hence there is no hard and fast rules with respect to initiation because it is also part of Non Vedic literature as well as stotras which usually does not need initiation.
But it is always the best to get it from a Sadguru.