They're all the same fish... Pet trade names are simply there to help sell more fish. Typically, in my experience, a Tiger Oscar has all red markings, in circular or random stripe type patterns. A Red Oscar will be solid red. A Common Oscar will be similar to the Tiger but have a lot less red with the markings being white, green, gray etc...
This is certainly debatable and I don't claim to be the authority on pet trade names (who is?). I guess my only real opinion on this thread is that I doubt the fish in question is actually wild caught.