Under the oil filter housing?
 

I have an engine oil leak just below the oil filter housing.It seems to come from a sending unit.Would it be the oil pressure switch?

What year and model?

I second that we need to know what you have to offer any sugestions.

it's an '83 SD right? i am thinking the oil filter housing to block gasket. or if it's the other side it might be the oil pressure line leading up to the instrument panel.
first thing to do is clean it up really well and re-examine.

83 300SD has an electronic oil pressure sending unit, so the line to the instrument panel would not leak, unlike a 123, which has a direct sensor and can leak. The sending unit could leak. Other alternatives are the oil filter housing to engine block gasket, but I would favor the oil-filter housing to turbocharger oil supply line. They can get brittle and break. They are sort of a pain to replace.