I have never kept lobster before so can't confess to be an expert. However, in order for lobsters/crabs to grow, they need to shed their outer shell every few weeks. This is a very vulnerable time for them which is why they always hide under rocks or in crevices in the wild, if they don't, they make a very easy meal because they are so soft. It wouldn't take much for an Oscar to cause great harm to your lobster at this time so I would strongly advise you to keep it somewhere else (a pan of boiling water would be my preference

) only joking.
You also have to bear in mind that lobsters are scavengers and have very powerful claws. One of the claws they use for grabbing and holding onto things, the other one is for cutting. Any fish could be seriously injured by the lobster, especially at night. This is another reason why I never recommend keeping this type of creature together with fish. And finally, there is a video somewhere you tube that shows an Oscar killing and eating a blue lobster. Just another reason why it would be a bad idea having one in with your Oscars