Any of the big 3 DCC systems PRRJIM listed above are great systems. A few of us on the forums use the NCE PH10. (You'll need the 10amp system for o gauge.) The power supply you have for DCS will almost certainly work for these systems as well, especially the NCE system. In fact, I simply use a cheap $20 20amp switching power supply on my system. I went with the PH10, which has a PowerCab with a cord, but that was because the system allows me to interface my own system to it, so I can develop my own app or RF remote control device. The PH10-R utilizes a wireless remote, and would be a better option if you didnt want to roll your own remote. (Because the cord gets old really quickly)
As far as decoders, everyone has their preference, but Loksound has a large number of European sound files available for you to listen to at the below link. (Almost 300 just for the European models) For O gauge, you would be looking at the "L" version. (Ie Loksound V4.0 L) They have larger decoders as well for the garden trains, so they service the entire spectrum of gauges.
