Common sources of oil leaks:

• Crankcase breather:
  The short piece of 14mm rubber tube that connects the top
  of the rear rocker cover to the air box, it is common for
  this rubber tube to perish resulting in oil leaks. The tube
  has a 14mm ID where is very rare apparently.
• The external seal of the head gasket sometimes leaks oil.
  Often cleaning and applying high grade epoxy can temporarily
  cure the leak, how ever if the faulty head gasket is causing
  poor compression or motor oil contamination then the gasket
  will need to be replaced and warpage checked.
• The electrical wire for the ignition system's pulse rotor
  that comes out the front side of the right crankcase cover
  (clutch cover). If the wire was pulled too hard then it make leak oil,
  you can thoroghly degrease it with a good quality degreaser and
  then pack in silicone gasket sealant in the rubber nipple and
  around the wire, then use some oil resistant heat shrink tubing
  to seal permanently.
• Rocker cover rubber gaskets are relatively easy to replace,
  see valve clearance.