While driving home it finally dawned on me that this might be the problem, from the data sheet:

"Fail-Safe Clock Monitor (FSCM) designed to detect a failure of the external clock source and switch detect a failure of the external clock source (LP,
XT, HS, EC or RC modes) and switch
automatically to the internal oscillator."


It may be that the external crystal isn't firing up and that is detected as a failure and the internal clock is automatically switched to. I will investigate and report back. This would explain why it seems that OSCCON doesn't work right.