Could be from anywhere. Check these things. Clutch cover gasket, bad dipstick o-ring, bad seal on the sprocket shaft, bad seal on the clutch pull arm, leak where the crankcase halves meet, or if worst comes to worst then the crankcase halves or clutch cover are cracked somewhere. Are you seeing an oil leak anywhere?
Check the cooling system anyway. Might be a bad seal on the impeller or bad seal where the clutch cover and cooling tube meet.