Until recently, we didn't even enter the OAT into the FMC on the 777 where I work. We have been doing this for a while.
The FMC knows that the Standard temp is, and will calc per that, baring other input. As long as you are at 20 degrees C, and at sea level, you are good.
What about ISA DEV?
What do you do for FMC PERF INIT? and T/C OAT?