My understanding is it would be 87.5% because the equipment is technically not available for production during preventive / predictive maintenance.
Below are the basics - I'll let the folks who use this a lot discuss your specific question of the definition of the A (Availability) factor.
OEE means overall equipment effectiveness. It is a method to find out overall effectiveness of equipment. It is obtained by multiplication of three ratios.
1. Availability ratio - Time for which equipment was available for operation divided by total calender period for which OEE is being calculated.
2. Quality Ratio - Quantity of "A" grade/Prime grade material produced divided by total production (Off grade+Prime grade)
3.Performance Ratio - Rate of production divided by Capacity of machine to produced.
Normaly OEE is presented in terms of percentage.
Elsmar Cove Wiki: Overall Equipment Effectiveness