O scale. But be wary when buying track and trains, as O Gauge is sort of a different thing. O scale is 1:48 in the US, 1:43.5 in the UK, and 1:45 in the rest of Europe. For the most part, any of those should be close enough to work, though you may occasionally get a piece that looks the wrong size, especially when dealing with items that are commonly seen with people. (For example, a dog in the wrong scale may be noticeable when placed next to a figure, but nobody will probably notice if a mine cart is a little big or small, since you don't see them everyday and don't have a reference point to compare to.) O Gauge refers to tracks made for the O Scale, but within O Gauge there are a number of different track sizes, representing different standards around the world for the width between rails. Usually they are denoted by modifiers, such as On3 or O-27. It's not a huge matter for models, but if you want track and a train to go on them, you'll want to be sure both track and train are of the same guage, or they won't work together.
Lionel (and K-Line) are sort of "brands" that don't really go by scale, as they sort of assume you're buying all Lionel stuff if you have a Lionel layout. However, both of those brands are roughly O-scale, and will work. Though, they are often a little simplified in design and may look too toy-like with the rest of your stuff.