First of all, you cannot reliably assume that any EXIF data at all is correct; all of it can be very trivially faked. Nor can you assume that the user even set the camera's time to anything close to the truth.
But if we rephrase the question as: provided that the user did set the camera's time to something they believed to be true, can you then assume that they intended it to be UTC?
Most users, when given only the option to set a date and a time, but with no timezone indication, will set it to their local time, not UTC. I certainly always did exactly that because local time is what I use.
I tested it on my photo collection. I have used many different camera models over the years. It looks like the only camera I've ever used that sets this "OffsetTimeOriginal" field is my most recent Canon camera (Canon EOS M50). This makes sense because there is a way to configure the timezone in its menu, which I don't remember from other standalone cameras. All the others (other Canon cameras, various compact cameras, various smartphones) do not set this field.
I would assume, in most cases, that DateTimeOriginal without OffsetTimeOriginal is local time where the photo was taken.