Your best chance for attracting orioles is right away in the spring. As they move north, they are looking for a stable food source and once they find it will nest nearby. I have had orioles come as early as April in Wisconsin, and it came and sat on the oriole nectar feeder for what seemed like a half hour, just drinking and gaining strength.
Orioles can be more timid than other birds, and you'll have luck starting by placing their new feeder near bushes or trees where they can quickly take cover. But don't put it under a tree or bush, oriole's then won't find it when flying overhead. Orioles like the color orange, so when purchasing a nectar feeder or fruit feeder, look for one with orange on it.
The recipe for oriole nectar is 6 parts water to one part sugar, heat it to disolve the sugar, just like you would for hummingbird nectar. Then, let it cool before filling your feeder.
There are several kinds of Orioles in the US, the most often visitor is the baltimore oriole. You may also see the Bullock's oriole, Orchard Oriole, Scott's Oriole or the Hooded Oriole. They are all members of the blackbird family.
Like hummingbird feeders, you may also find bees, wasps and ants taking over your oriole feeder. Because orioles don't have the long beaks that hummingbirds have, many of the unique bee-proof hummingbird feeders won't work for Orioles. So, if you have bees take over your oriole feeder, take it down until the bees go away, then hang it back up. (don't attempt removing it when it is full of bees!)
For ants, try using an ant moat, like the one from Barn in the Sticks, the Nectar Protector.