Can I renew my OA visa in January and switch to an O visa without leaving Thailand?

That's exactly how the OA visa works, you get stamped in for a year OR as long as your mandatory health insurance is valid for (up to a year)

Nope, you can't renew ANY visa inside the country nor can you change to an O visa from an OA entry stamp 😕

You COULD apply for a yearly extension of stay by meeting the in country proof of funds requirement AND having a new yearly insurance policy with the 3 million baht mandatory coverage.

IF you want to get off the OA and switch to an O visa that doesn't require insurance you'd have to leave the country AFTER your OA visa (not your entry stamp) expires, re-enter the country on a visa exempt entry then apply for an in country Non-O visa from that and after that the year extension.