My recent dissertation treats the history of OM in Vedic texts and ritual:
Gerety, Finnian M.M. 2015. "This Whole World is OM: Song, Soteriology, and the Emergence of the Sacred Syllable." PhD. diss., Department of South Asian Studies, Harvard University.
Chapter Two deals with passages from the Saṃhitās (including your passage VS 2.13 with partial translation, p. 55), while Chapter Four discusses previous accounts of the syllable's origins and early history (including those of Keith, Parpola, and Hock). In a nutshell, I argue that Sāmavedic texts and rituals played a decisive role in fostering OM's emergence as a "sacred syllable"; as far as I can tell, the earliest actual attestation of OM in Vedic texts is as a stobha in the songs of the Sāmaveda Saṃhitās. However, because OM is implicated in the recitations of all three Vedic liturgies, the syllable was likely added to mantras in ritual performance much more frequently than the evidence of the Saṃhitās alone would suggest. Thus, a comprehensive assessment of OM in the Vedas also requires reconstructing recitation and performance with the help of the Śrauta Sūtras (see my Chapter Three).
Visiting Assistant Professor of Religious Studies // Brown University