This assumes you have the scale wheels. For 2-rail track, Atlas-O 2-rail track is Code 148 (.148" rail height), plus they make a couple of turnouts and crossings. Micro Engineering also makes Code 148 2-rail track for O scale, plus Code 125 and Code 100, but you'll have to make your own turnouts and crossings or have them done by a switch maker for the Code 125 an Code 100.
If you have MTH hi-rail wheels, the flanges are cut a hair smaller than the older hi-rail flanges and usually clear on Code 148 track, but turnout guard rails will have issues because the flange back-to-back distance is less than on a scale-wheel configuration (to support the wider hi-rail treads).