LeoLily, Something else I thought is that the O-rings sit in two grooves in the block around each metal pipe. The pipes protrude past the block & insert into the oil cooler holes by about 3/4". The pipes looked to be welded in place on the outside surface of the block....if I remember correctly!
I'm thinking that your bolt maybe just loose or the welds maybe cracked??? Diagnostics would be a bit tough just by eye. Maybe wipe down the area very well & run the motor & see if a tissue test could tell you if the oil is coming from under the block which would mean the O-rings or a loose bolt. If the oil is running off of the block and then down the side of the cooler then I would suspect the welds. I use a bit of paper towel to check the area around fittings for leaks as even if the leak is small the paper towel will wick up spillage at once.
Tom