Do coaches get better with experience? So I screwed up.... my O-line coach had a 79 rating, so without looking at other ratings, I promoted him with a 5 or 6 year deal. Then I noticed he has a 63 rating as OC lol. Will he get significantly better? or can I demote him?

Yes, but you would be using a guy that would be far below average for 5-10 seasons before he would be where he should be, also coaches can progress downward as well. The season is almost over though in 28. Hire yourself a decent OC in early free agency which is the only time of the season/offseason in which you can hire other teams position coaches for promotions to HC or Coordinator.