Larry, it makes a difference if the OAT is hooked up to an EFIS or an EMS.
If an EFIS, push a center button to open the menu, then MORE->SETUP->UNITS->MORE->and change CELCUS to FARNHT.
If an EMS, push a center button to open the menu, then MORE->SETUP->GLOBAL->SEL->UNITS. You will see that there are separate selections for ENGINE TEMP and OAT TEMP.
-Dynon