There are multiple types of Oregon grape... I wonder which type you planted.
Tall Oregon Grape, Mahonia aquifolium prefers drier sunnier conditions though it handles semi-shade okay.
Low Oregon Grape, Mahonia nervosa on the other hand grows in forest conditions and loves shade and doesn't do well out in the open. There is also
Creeping Oregon Grape, Mahonia repens which grows in dry and cooler areas. Here in western WA we have the tall and low Oregon grapes but I haven't seen the creeping one and it sounds like it's generally found east of the Cascade Mountains.
There are also several cultivars that are sold in nurseries sometimes under the name Oregon grape. So this is one case where knowing which one you're getting is important since they like different conditions.
Also, Oregon grapes are very sensitive to
root disturbance. Often they can be killed when planting if their root tips are disturbed too much.
I've often had to dig holes that stretch out a fair bit horizontally to make room for their
roots. This is especially the case with the tall and low Oregon grapes which will both spread
underground and send up new growth away from the original plant.