The error message would seem to imply that the level links against a newer version of op2extra.dll than is currently available. I'd say try to find an updated op2extra.dll.
Of course, it could be the other way around, and you need an older version. Basically, you need a function that doesn't exist, either because it wasn't added yet, or it got dropped (or renamed) in the version you have. Probably because it wasn't added yet, in which case you need a newer dll.
Version 1.3.5 does ship with op2extra.dll, so it's possible it replaced an existing dll with an older one, which could cause the error. Although, I would have assumed that Renegades comes with the dll, so installing Renegades after installing Outpost2 should probably set you up with the right version.