Yes, it is possible to sell ONGC / Oil and Natural Gas futures without buying them. But, you must match it with a similar trade in the opposite direction. This is known as flattening a position in futures trading. Only then will you be able to realize your profits.
You don't have to hold on to a ONGC / Oil and Natural Gas futures till the expiry date. In fact, most traders are known to exit their positions in futures contracts before expiry. The margin you've deposited is used to settle any profits or losses made until the date of exiting the futures contract.
Trading a futures contract of ONGC / Oil and Natural Gas on the same day is known as intraday futures trading. This allows you to buy or sell a futures contract within minutes or hours without having to hold a position overnight. Your intraday position will be squared off automatically by 3.10 PM.
ONGC / Oil and Natural Gas futures come with an expiration date. A majority of traders are known to exit their futures positions before expiry. Long story short, this means you can hold on to any futures contract till expiry. After that, your position will automatically be closed. That's not all. Your funds will be adjusted based on whether you've made a profit or loss.