Hi Kestrel,
I cant answer your autopilot question but I will do my best to explain the difference between OAT and TAT:
OAT (outside air temperature) = the temperature of air under normal/undisturbed conditions outside the aircraft. This is known as the 'Static Air Temperature'
TAT (total air temperature) = as the aircraft speed increases, the air close the aircraft is compressed and because of this compression the air is heated. This is known as the 'Ram Rise'
If you add the 'Static Air Temperature' and the 'Ram Rise' together you get the Total Air Temperature (TAT)
I hope this helps.
Good luck with your flying!