Most GPIO pins when selected to be outputs are push/pull. They either pull up to Vdd or pull down to Vss. When outputting a digital 1, the pin is pulled up to Vdd as if an internal switch were connecting the pin to Vdd within the microcontroller. When outputting a digital 0, the pin is pulled down to Vss (0 V) as if an internal switch were connecting the pin to Vss within the micro.
An open collector output normally implements only the pull-down part of the switch. When set to output a digital 1, the pin just floats. Some MIcrochip PIC devices had a pin that was open drain and it used to catch people out as when they tried to drive the pin high (e.g. to light an LED of which one side was grounded), it never would source current into the LED.
As this is fundamental information for correct use of the pin, the data sheet should make it very clear if it is open collector output. Otherwise it probably won't be. And the advice given by the previous respondent about not exceeding the voltage of Vdd is good - even if a pin is designed as open collector output, it will most likely have the same voltage limits as the rest of the chip and also have protection diodes internally to the chip supply.
If you really need open collector, you could add an external transistor and then drive this with the microcontroller pin.