Right, open steam, go to your games library, right click on Omsi, select ‘properties’ then go to ‘Game files’ (it should appear on the list on the Properties box), then choose ‘Verify the integrity of your game cache (or files, can’t remember which one). As I said before, Steam will automatically search for any missing files and automatically reacquire (re-download) them. After doing the search a box will come up saying ‘x (the number) amount of files failed to validate and will be reaquired’.
I did that and it said about the files. So now do I just leave it to download?