Technically 50 EU is US 34, but the one pair of OL trousers I have are that very size and they run smaller than any other pants I've ever had. They are about 16.5 inches across the waist, with the front and back lined up perfectly flat. I keep meaning to find a tailor to let them out a bit.
Theoretically a perfect size 34 should measure 17.0 inches across the waist, but I find most from other brands are 17.25 - 17.75, so if I ever buy more pants from OL I will probably size up to 52 and hope they fit the same as the rest of my pants.
This may be common knowledge in this thread or for all I know others may disagree. Keep in mind my sample size is one, as I've only ever tried on the pair of OL pants I own.
OL sizing is all over the place. A couple of seasons ago their chinos fit tiny and last year their wool trousers came up small, but for the most part their legwear has fit true to size over the years. I'm a 34 waist and own mainly size 50s with a couple of 52s thrown in.