Diagnosis
This isn't necessarily a defect, but a common assembly issue. The oscillation mechanism is often tight from the factory, or the gear alignment is slightly off, causing binding. It needs to be freed up.
How to Fix It
With the fan unplugged, manually help it through its full range of motion. Gently grasp the top of the fan and slowly rotate it left and right as far as it will go, applying steady pressure. Do this several times. You might hear or feel a slight 'pop' or release as the gears settle into place. Then, plug it in and turn on oscillation. It should be smoother.
If That Didn't Work
If it's still binding badly, there might be a piece of packaging debris or a mis-molded plastic flash in the gear track. You'd need to open the center column to inspect and clear any obstructions, as described in the noise problem fix.
Heads Up
Don't force the oscillation by hand while the fan is powered on. You can strip the plastic drive gear instantly. Always unplug it before manually moving the oscillation assembly.