@Michal ***********
you're on a year ENTRY STAMP, the VISA is a totally different thing and you can't switch off that entry stamp to another visa type or extension other than for the reason you have the OA visa.
You would LEAVE get OFF this entry stamp, GO buy a new Non-O visa based on marriage at one of the consulates I mentioned then enter on it and get stamped in for 90 days
you aren't gonna do anything that you are wanting to do inside the country on that year entry stamp from the OA visa. You HAVE to exit