PicBasic or PICBASIC Pro?

I guess you talk about OSCCON register @8F.. right?

you can't write to bit IOFS.. well you can but it will never do something.. it's a read-only bit. This bit tell you when the internal OSC frequency is stable.. not much

Let's say you want to set frequency to 8MHZ

PBP:
OSCCON=%01110000
OR if you want...
OSCCON.6=1
OSCCON.5=1
OSCCON.4=1

PicBasic... i can be wrong...
POKE $8F,%01110000

HTH