1. define OSC 20 does not set the oscillator, it simply informs PBP of the oscillator's intended frequency so it can adjust it's timing.
2. Your chip has a fail safe monitor in case the oscillator fails, so if it fails it switches to internal osc. It appears OSCON sets the FSOSC's frequency, not the Primary OSC .
I would try shutting off the fail safe clock monitor, I would guess then it won't run at all. You may have a dead crystal, too much capacitance on the crystal or a chip with a damaged oscillator. If you have an oscilloscope you could test . . . HTH