Well, depending on the intent, I guess
If the UTAU has a persona to it, an avatar, I guess you could call it, to illustrate the voice, I guess it can be both an OC and an UTAU. But some people create voicebanks without planning on making a design, a faceless voice like VY1 and VY2, that's I guess,, Just an UTAU, which is nothing to be ashamed of, not all need a design to be unique, but I wouldn't really call that an OC. Just an UTAU voicebank.
But if someone makes an "UTAU" character without ever planning on making a voicebank, or only describing the voice, that's just an OC. An UTAU-based OC, which is essentially the same as just a fanloid, just a fan-insert into the world of VOCALOID or UTAU, which is... Okay, I guess, I guess designing vocaloid-styled characters is pretty fun, but. I've never fully understood the part if someone uses them as just a character to insert themselves into the fan-assumed lifestyles of the VOCALOID and UTAU characters,,, but it would be nice if everyone had the ability to voice their characters, in my opinion.
But uh those are just my jumbled up thoughts. //please don't hate me