The oil pressure idiot light is a pressure switch that turns on a light on the dashboard. Wire a standard automotive relay with the center pin on it to the light so when the light comes on it cuts power to the blower
A relay without that center pin would only let you have heat when the oil light is on.
87a is on until the relay is energized then 87 is on. so 30 and 85 go to the idiot light and 86 and 87a splice into the blower motor.