If i had to describe what I felt was random, I'd say they were questions that I would not have anticipated to be on the test.
Most questions I got on the OAT were ones I had seen in some similar form beforehand during my practice, and I learned how to approach those - i.e. what equations I had to manipulate, or at least I knew what they were asking (not that I knew how to do all those, but I felt they were fair).
The random questions to ME seemed to be those that I hadn't encountered in any form at all before, and I had no idea of how to tackle or answer those. You'll see what I mean when you are studying and taking practice exams because those pop up there too. Most of those just require you to think in a different way, but if you know your stuff really well this shouldn't be a huge problem. Sorry if my attempt to explain this makes no sense!