Yes. You can file RTI seeking certified copies of your answer sheet / OMR for UPSC Prelims Examination 2018.
UPSC was established on 1 October 1926 as Public Service Commission, it was later reconstituted as Federal Public Service Commission by the Government of India Act, 1935; only to be renamed as today's Union Public Service Commission after the independence. UPSC is a Public Authority under the provisions of Section 2(h) of the RTI Act 2005 and hence comes under the preview of RTI.
A RTI application seeking answer sheets should be addressed to the designated PIO with a copy of the admit card. As RTI application to UPSC is filed online on the government portal, the wait time for getting a response may be expected to be a little less than usual.
To know the further procedure to file RTI Applications for Central Government Departments please refer https://onlinerti.com/how-to-file-rti-online-for-center