I just went outside and looked at our Oregon Grape and there are green terminal buds at the tip of each stem. There are also axillary buds between the leaves and stem. Bud break shouldn't occur until next spring (March to May depending on cultivar and micro-climate). If the buds break early, it may be due to some transplant shock, and you would likely see damage to the new growth due to frost or extreme freezes. Otherwise, it sounds like the plant dropped some leaves due to some transplant stress but is recovering. You should see new growth in the spring. For more on the botany of Oregon Grape, refer to: https://landscapeplants.oregonstate.edu/plants/mahonia-aquifolium

Thank you so much!