At least within the PADI system, the AOW course is designed to give you an exposure to 5 different dive types (Navigation and Deep are required, and 3 elective dives are chosen from Altitude, AWARE-Fish ID, Boat, DPV, Drift, Dry Suit, Multilevel-Computer, Night, Peak Performance Buoyancy, Search and Recovery, Underwater Naturalist, Underwater Photography, Underwater Videography, and Wreck). Depending on the facility, you may or may not have much of a say in which of the elective dives will be conducted.
If you're relatively new to diving, taking AOW to broaden your range of diving experiences might be a good thing to do. Usually, the specialty courses go into more detail than what's covered in that dive's portion of the AOW course.
If you decide to continue on and complete the specialty courses corresponding to the dives from your AOW course, the dive from AOW may be counted toward one of the dives required by the specialty course.
And just to clarify something from your post, again, at least within PADI, Nitrox is not part of AOW, but a specialty only, and Rescue is not a specialty but a certification level (which has AOW as a prerequisite).
Hope that helps.