If oct scan shows slight progression of glaucoma, how often should it be repeated in order to look for trends? pressure is stable & visual field good.

Once or 2x per year: Oct data is helpful but not exclusive for determining glaucoma stability. I do find it supportive and often correlate-able to the visual filed changes. When the oct is showing change, yet the visual field is not, it's often difficult to decide how much of a change is significant to warrant more treatment. And of course how much rnfl change is normal vs pathologic has to be factored into the analys.

Confirm change first: Check visual field to correlate finding. Typically repeated once a year once glaucoma stabilized, can be sooner if the pressure is high or making a decision to advance therapy usually smallest interval to detect change if the iop is not high is still months.