O27 is not a scale per se. Lionel tends to build stuff to work on their curves. Which is fine. That is who they are. Hence most if not all 027 cars and equipment are closer to S scale (3/16 inch to the foot) than O Scale (1/4 inch to the foot).
Most kits are O scale. There are a few old ones out there that are actually on the big side (17/64) from the 30s. I would say most wooden/metal kits from All Nation, Athearn, Reynolds, Main Line, Walthers etc are O scale.