Try holding the plastic catch open and banging the bonnet with the side of your fist at the point where the catch is this sometimes works (unless u have very long arms u need someone else to help!) The cable may just need adjusting. ONce u got it open have a look at the catch and you should see a screw holding the release cable in place. Loosen the screw and pull the cable tight (not too tight, just to take up slack)
If the oil light is coming on it means there is low oil pressure so engine could suffer suffer damage almost immedialty. OIl lights tend to be warnings that your engine is about to seize up rather than a low oil level.