Does anyone know whether the RC questions in OG 11 are listed in order of difficulty? I know this is the case for CR and SC but wasn't sure about RC.

The OG doesn't say that they are, so I see no reason to assume that there would be any such difficulty gradient.

More to the point, the problems are (obviously) clustered by passage: all the questions that go with any particular passage are given together. Therefore, the only way that the questions' difficulty could generally increase would be if the passages themselves, along with all their questions, got 'harder' as you went along. That's almost certainly not true.