I have noticed an oil leak from the exhaust side of my turbo, can anyone tell me what might be causing this before I start to tear into it?

I have been running stock exhaust but it ends at the union, so no muffler just a straight pipe.

I am thinking of putting 3in pipe from the turbo back. I just want to know if there is anything I need to do to the turbo before I do the 3in pipe.

Thanks Jon