That's what I use in the GPSDO in one of my projects. The internal voltage reference powers a 10bit DAC that we dither to tune the OCXO. If the microcontroller has a voltage rail just for it's internal DAC, that would be the best way to run it. You could drop the opamp and voltage reference from the design and just keep the RC filter.
The opamp would still be a good idea no? That RC filter output will be very susceptible to noise and we wish the OCXO tune input to be as stable as possible. When I built one of these a friend had me route the opamp output as a guard ring around the entire DAC to VCXO tune net.
