Uncertain. Has been linked to both Ancient GreekΟ(O, “omikron”) and Ω(Ō, “omega”), as well as Runic ᛟ(representing long or short /o/). Snædal (2015; see references) favours the first derivation, interpreting it as being (like the letter 𐌿(u, “urus”)) an adapted allograph of Greek omikron.
Snædal, Magnus, 'Gothic Contact with Latin: Gotica Parisina and Wulfila's Alphabet', in: Askedal, J.O. and H.F. Nielsen ed., Early Germanic Languages in Contact (Amsterdam 2015) pp. 91-108, especially 103.