“uxo” is not really a romanization, it’s just a way to input it into an IME (Input Method Editor).
Prefixing a kana with “x” in an IME will generally get you the small version:
xa → ぁ (vs あ)
xya → ゃ (vs や)
xtsu → っ (vs つ)
Actually I would tend to write “who” instead of “uxo” in this case, which also results in ウォ - the reason it doesn’t have “wo” as input sequence is because that is already taken, for ヲ.
But “wo” is a correct (and I suspect by far the most common) romanization of ウォ.