If its stuck together you cooked it too long. I look at what it says on the packet and take off two minutes. Make sure water is boiling when you put it in and dont use a lid.
its not caused by cooking for too long as it sticks together almost immediately it goes in the water. the best bet is to agitate it with a form whilst cooking
Put simply, you need to use lots and lots of boiling water with your spaghetti - in as big a pan as you have. Oil doesn't work whilst cooking - it's fine to dress the cooked spaghetti.