Hi,
when the supply voltage is 3.3V, the MCU is not 5V tolerant even if a pin is configured as open drain. The datasheet explicitly states that open drain pin must be pulled to VDD. And Vih spec says that input voltage can't exceed VDD+0.3V. If an external circuit requires 5V, the only options are either to use 5V supply voltage for MCU or use some voltage level translator.
Regards,
Lukas