Sri Sitaramdas Omkarnath was definitely from the Ramanuja-Ramanandi sect as evident from the Guru-paramparaa sloka He writes in the preface of His 'Sri Sri Krishna-Nama-Mahima'
Srimad RAma-samarambhAm srirAmananda-madhymAm SridAsarathiparyantam vande Guru-paramparAm
English Translation: I salute the Guru lineage starting from Sri RAma and coming through Sri Ramananda upto Sri Dasarathi Deva (His Guru). His tilaka also reveals His sect (image enclosed).
However, in His 'Prapanna-Pathika'(Jnana-Marga), He writes
Brahma eva aham. I am not the body. I am Brahman...If the burning fire is cold, only then this world exists--OM... If the king of the snakes be killed by the horns of a rabbit, then this world exists--OM...Only I exist, the Ever-unchanging Ever-Blissful I exist...Asmi Asmi Asmi.....
It became possible because He Himself realised the Ultimate Truth that way. His philisophy is known as the 'OMKARA-VAADA' -- which says that all mantras merge with the Anaahata naada 'OM' and OM ultimately merges in the Supreme Self. He named the Saguna Brahman as 'Apara-Pranava' and the Nirguna-Brahman as the 'Para-Pranava'. He writes that both are simultaneously Ultimate, depending upon whether the aspirant follows the path of devotion or of knowledge.
Who merges with 'You' is a bhakta and who merges with 'I' is a jnAni.
So He ultimately is speaking from His own realisation rather than stating the views of his own sect. He was highly revered by many saints of different sects including some from the Ramanuja and the Ramananda sects.
He mentions that all the doctrines are simultaneously true as none can fathom God who is beyond speech, mind and intellect.
Reference: Omkarnath Rachanavali, Omkarnath Mission, Vol 14, page 3- 204.