What happens to my open short option position on the day of expiry?

On the day of expiry Short option positions that are In-The-Money (ITM) and held under NRML will devolve into the corresponding futures contract, subject to availability of sufficient margin.

And Short option positions under MIS (ITM or OTM) will be compulsorily squared off by RMS as per the applicable cut-off time, subject to availability of market liquidity, and on a best-effort basis. These positions will not devolve into futures.