Oh boy, you're going to have fun!
Where are you going to be on the Osa? A few kilometers from Bosque del Cabo is an area right on the road frequented by scarlet macaws. With patience you can get some great shots. Bosque del Cabo itself is a great place for monkeys, curassows, frogs, coatis and a host of other treats.
You will definitely want your 500 as well as a macro lens.
In the Selve Verde area check out the Nature Pavilion for some great bird photography. The owners are very helpful and enthusiastic. La Selva Biological Station is worth a visit as well.
The Savegre area is good for quetzals and great for hummingbirds.
Rancho Naturalista is another great spot for birds.
I'm sure there are many other good places, but these are the spots I have personally had the best experiences.