I left 2 oleander bushes outside this winter, they seem to still be living. They have a lot of dead leaves on them. Should I cut them back or leave them as is? I live in Gaithersburg, Md.

I'm definitely not a gardener, but I wouldn't cut anything until after the last freeze. (As I write this, the temperature here, just outside of St. Louis, is 27°F.)

Be careful with oleander, as it is toxic. Ingestion of even small amounts can kill. Here is the Wikipedia entry, which may be helpful, but I agree with Dorian: don't prune just yet. Once the danger of frost has passed and you start to see new growth on your plant, go ahead and trim. Do it when there is no dew on the plants.
Also, the Old Farmer's Almanac can be helpful for pruning guides and suggestions, though I don't see anything specifically for oleander.