I hace a quest in my log book that simply has an "O" in the slot. I don't know if I touched something or if this is the start of a new quest... Any comments?

Could it be the clue from the obelisk? Every time you touch a new obelisk, you get a piece of the secret message for finding some legendary treasure. But I think these letters appear in the "notes" section...

afaik it's just a visual bug, that should not affect your game.

OK, just was interested. I don't like to miss much when I work through a game, then it's time for a new one. Thanks guys

It's a known bug that pops up in your quest log after a certain event (don't remember which one any more), don't heed it any mind it won't cause any trouble :)

You get it after doing the quest for both sides during the war before getting the horn, I think.

Yup. After completing the quests for both Queen Catherine and King Parson, the next time you enter Castle Harmondale, you'll get the 0 quest.

It doesn't really affect your game, though.