How do I prevent this?
When I am making marinades or making gardinar the olive oil turns to a solid.
Does anyone else have this problem?
No, olive oil should not be refrigerated after being opened. As for length I wouldn't keep it over 6mos, but mine never lasts me that long anyways. So long as you're careful not to infuse it with any raw products (especially garlic) it will keep for quiet some time.
"Mother Nature is the true artist, the Chef is merely the technician"