So, you want to add track to the oval that came with your O gauge train set. You visit a local hobby store to buy more, and you see a dizzying array of track that looks vastly different, all of it labeled O gauge. What’s up with that?
There are a number of manufacturers of O gauge model railroad track, and some manufacture more than one style of track. All O gauge track features the same width between rails (What's the difference between O gauge and O-27 gauge?) so your O gauge trains will run on any manufacturer’s track, but not all manufacturers’ track will work together well. Each manufacturer has a method of joining track pieces together, so to use track from different manufacturers often requires a special adapter piece.
If you already have a lot of one style or brand of track, it'll be cheaper in the long run to stay with that, but if you are just getting started, and you only have enough track to make a simple loop, it’s a lot easier to switch track brands now, rather than later, after you have bought hundreds of dollars worth of additional track (see what expanding your layout can cost on ourComparing Prices of O Gauge Track Page). Ask you local hobby shop about their opinions of the various track brands and styles available. They all have pros and cons, so this is a good time to study them and see if you want to change.