The EU sanctions aren’t meant to prevent anyone from buying Russian oil at all, but rather to limit how much Russia can profit from selling it. If Russia could be magically prevented from selling any oil it’d make prices enormously higher for everyone across the world and functionally cut off the poorest countries from being able to buy any. Assuming you could get this to happen, it’s rapidly going to turn most of the world against the EU and have far wider-reaching consequences than just hurting the Russian economy. The plan instead is to force Russia to sell their oil for cheaper than they want to and make it more expensive for Russia to actually move the oil, ideally resulting in Russian oil staying on the market but cutting the actual money made substantially. If India buys it cheap, that actually works fine.