To unlock each ending, you must finish the game a certain way. For ending O, you'll need to:
Optional: You can start at Bracciano (level 2 below)
Level 1 - Keep right.
Intermission 1 - Nothing.
Level 2 - Win the race, keeping right.
Intermission 2 - Leave Olga ("better to be apart").
Level 3 - You can ignore the priest. Pick up the hitchhiker and ask him about fireflies at the farm and what Pasolini had written about them and just be nice to him. Make sure to not select the "born in the wrong decade" dialogue option!
Finale - Nothing.
Video courtesy of Dernop