An amazingly accurate rule of thumb (works up to FL400) for working out OAT from TAT is to take 50 off the Mach number and subtract this from TAT. I think a 319 cruises at .78 so if the OAT is -65 (sounds a bit cold for FL330?) add 28 (50 less than Mach no.) and TAT would be -37 within a degree or so.