The only reason I could see why you would decrease the amount of watering that you give your oncidium is because the plant will be entering a minor "resting" stage after blooming. This is not dormancy- just recuperating from the major energy expense of blooming.
This being said, I have found in my personal experience and with what I have heard from other growers, that the plants will enter a minor resting stage in which they do not take up as much water- because they are conserving energy. When they aren't using up as much water, the mix doesn't dry out as fast, which should lead to watering less often.
My best advice is to just monitor your plant and water as you would normally - if it's approaching dryness, give it water, etc. The main thing will just be to make sure that you watch how quickly it is drying out (or not). That will affect your watering schedule until it enters full growth phase again.
Does that help? PS- wrinkly PBs is a major indicator that your oncidium is thirsty...
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