The Denmark international had six months with OM before returning to Brighton during the winter market.
O’Riley told The Argus: “This season feels like a lot, but not a lot at the same time.
“I played a couple games with Brighton at the very start of the season here, and then I went to Marseille, which happened quite abruptly.
“I had a really good experience there, very challenging in many ways. It was very demanding.
“Naturally, a club with a lot of pressure because the fans there are so passionate about their club.
“I was playing in front of, what, nearly 70,000 every game, in very hostile atmospheres.
“So, mentally, I think it has put me in a really good place coming back here and just being able to deal with different environments, different manager, of course, very different style, different culture, different people.
“Now I'm back at Brighton, and of course, I'd like to be more involved than I am at the moment.
“I feel like I'm able to do so. I feel like I'm in a really good place, that when that time comes, I will be ready.”