How OEE is calculated
OEE = Availability x Performance x Quality. Availability is run time divided by planned production time (it captures downtime). Performance is actual output versus the ideal output for the run time (it captures slow running and small stops). Quality is good units divided by total units (it captures defects and rework).
The headline OEE number matters less than the three components: they tell you whether you are losing capacity to downtime, slow running, or defects, so you know exactly where to focus improvement.