First of all check the oil drain plug,some times it got slipped due to over tightening or cross threading,if it is o.k then check the chamber's or oil sump's packing,if it is ok then check the main oil seals installed behind crank pulley and flywheel,if u found oil seepage or dripping from engine and gear coupling, it means gear oil seal or main engine seal is leaking.
Note: Before trace out the leakage, it is necessary to wash out the engine completely to find out the exact location.