Oil leaking from oil cooler.

Hi, my 1996 1.8T AEB engine code is leaking oil, i thought it was because the crankcase breather system was clogged up but i have just cleaned and replaced some parts and it's still leaking.

I jacked it up and looked underneath with the engine running and it seems to be leaking from the oil cooler to block joint, how do i stop the leak, is there a gasket that needs replacing or can i do the cooler up tighter (how is it held in place) i know when the filter is undone there is a large nut below, is this the one to tighten????????????

Ok, sussed it, replaced the thick rubber o ring between the cooler and the block, no more oil leaks!!!!!!!!!!!!