Interfacing open-collector optoisolator to 3.3V microcontroller
That open collector output V o is not pulled up internally according to the datasheet. Zero mention of it, and every diagram shows a simple npn open collector. 

Power the VCC at 5V, and pull up V o to 3.3V vía a external resistor or your microcontroller internal pull-up resistor.

Worst case, use a voltage divider to bring the signal at V o down to 3.3V.