I've boxwood, azaleas, a gardenia and a camellia planted near my oakleaf hydrangeas and the combination seems to work well. The oakleafs are underplanted with hellebores for winter interest. (I should mention that the oakleafs are in a front foundation bed.) The neighbors across the street who are the only other people in the area with oakleafs, have a nandina in front of theirs.

The other combination of shade shrubs that I like, unfortunately doesn't work in PA (at least from what I remember of the winters there): azaleas and camellias.

Monrovia's website might have some Ideas for you.

Given where your oakleafs are, I would think ferns, hostas, or evergreen shrubs of a woodland nature would work.

Good luck,
Mark

Here is a link that might be useful: Monrovia

I just put a single Camellia WInter's Fire with three Mahonia Japonica Beali as foundation plants. Looks pretty good. It has the advantages of being evergreen without looking too formal and boring.

I put my oakleaf hydrangea with a itea virginica. It should look great for the same reason as Fothergilla will - fall color.

I've got a row of 5 dragon lady hollies capped by a Sciadopitys verticillata -- Japanese Umbrella-Pine. Great contrast and evergreen.