They usually say that getting placed in Off campus is slightly difficult compared to that of On campus. More often than not, the reason can be because there is relatively less competition in On campus drives. Only your college mates participate along with you. Whereas the situation is different during Off campus drives. Students in huge number from different colleges attend the off campus drive.
But that should never be the demotivating factor. Prepare fully and give your best in both On and Off campus placements. Nothing can beat hardwork.