The APIs themselves are pretty similar. The biggest difference is in the scope/goals of the projects.

OT defines an API that various tracers can implement while OC ships a single implementation that tracing vendors can export data from.

OT is focused on traces, OC aims to support move observability signals. Traces and stats are available today. Logs and error tracking are on the roadmap.

There’s a few other differences, but those are the main ones.

"Open tracing" is a bit too opinionated by be universally useful. For example opentracing makes sampling decisions at the end of the span, not the beginning. Because of this you always pay the cost of span annotations even though the vast majority of spans are discarded. I think it's interesting that opentracing descends from the author of Dapper, and opencensus is from Google itself, and yet they've diverged on this and other details. Google of course just uses Dapper.