A quick Google of "repot Oncidium",produced this link among several, with similar advice: ttp://www.orchidsgardencentre.com/oncidium.html. Based on that advice, repotting now, with a shoot growing, should be ideal.
Sorry - it was late when I posted. I should have mentioned that I have only a few Oncidiums, but when I repot them, I also aim for this growth stage. This is similar to the best time to repot a Cattleya. Of course, if a plant is in desperate need, repot any time.