what benefit will I get if I am from OBC Category for UPSC Pre?
The actual cut off in the Prelims is disclosed by UPSC only after the declaration of the final civil services result. So the cut off for UPSC prelims 2018 will be known in May/June 2019 after the announcement of the civil services 2018 results.
Meanwhile you can know predicted cut-off for the 2018 Prelims from the different answers on this page.