Diagnosis
The leak is almost certainly from the blade assembly gasket (a black rubber ring). It's either worn out, cracked, or wasn't seated properly the last time you took the blades out for cleaning.
How to Fix It
Unscrew the blade assembly from the bottom of the pitcher (turn it counter-clockwise). Remove the black rubber gasket. Clean the gasket and the groove it sits in thoroughly—a tiny piece of food debris can break the seal. Re-seat the gasket firmly in its groove, then screw the blade assembly back on hand-tight. Don't use tools; overtightening can crack the plastic.
If That Didn't Work
Inspect the clear plastic pitcher itself, especially around the threaded area where the blade assembly screws on. A hairline crack there will also cause leaks and means you need a new pitcher.
Heads Up
The gasket is a wear item. If yours is flattened or stiff, it needs replacing. You can order a replacement gasket (part# 009927) directly from Oster or appliance parts sites.