Because of compressibility the measured IAT (Indicated Air Temp) is higher than the actual true OAT. Approximately
IAT=OAT+K*TAS^2/7592
The recovery factor K depends on installation and is usually in the range 0.95 to 1.0 but can be as low as 0.7
OAT =(IAT+273.15)/(1+0.2*K*M^2)-273.15