I work for an assisted living facility, I am an attendant full time and just recently have been offered a position in the office p/t, but I am unsure of how te law works for overtime and difference in pay.

This would all depend on your company's policies if you would get paid separately. If you punch a time clock for one job and not for the other job, that's the difference right there. But if you punch different time cards for each job, then you should be paid for each job performed separately.