Diagnosis
The motor is overheating because it's being overloaded. This model has a thermal cutoff, but the burning smell is the insulation on the motor windings getting too hot.
How to Fix It
Stop using it immediately and let it cool for at least 30 minutes. For future blends, you're likely overfilling it or using it for too long on high speed with thick contents. The max run time for heavy tasks like nut butters or frozen fruit is about 60 seconds. Use the pulse function in short bursts instead of holding a high speed continuously.
If That Didn't Work
If the smell happens even with small, light loads, the motor bearings might be seizing or the cooling fan (inside the base) could be blocked by dust or debris. Unplug it, turn it upside down, and check the vent slots for any blockage.
Heads Up
That burning smell is a serious warning. Repeated overheating will permanently damage the motor. If it's still under warranty and does this with reasonable use, contact Oster.