Al, If you were installing decoders in HO, plug and play or even hard wired, you'll have relatively little issues in O Scale. In fact, I think you'll find it's easier in O Scale. 
With the decoders available today, screw terminals, etc.. less soldering is required, even easier to get a locomotive converted to DCC. Don't be afraid, take your time and I think you'll be surprised at how easy it really is, especially with that Atlas GP15. Did your GP15 come with any wiring harnesses ? Often DCC ready Atlas locomotives come with 2 wiring harnesses that plug into the Atlas dogbone shaped board that use in their locomotives. One harness has pickup and motor leads, while the other is primarily for lighting. Remember at the core all you have to connect are the left and right rail pickup wires and the motor leads to the decoder. Once you have those you can test your work and proceed from there.
All I'm trying to say if don't be afraid to try an install. There are numerous threads on this forum and others around which decoders guys are using now for installs, almost cant make a wrong choice, and go for it.
