He's sticking to lines or edges of things. The stripe, the fence, the rail. He's orienting himself to those things. That, and he looks like a cute little boy out exploring the world in all of them.
Oliver is dreaming of being a tightrope walker and he's just waiting for the right time to join the circus? :-p
They all have me wishing that I knew what Oliver was thinking about those lines he is observing so closely!