Sam Ting Wong,
You Sir, are a complete moron.
An O day is not a day off, my O days are regularly converted to duties. You cannot plan anything on an O day because it is a form of reserve. Call 8929 and you O day is now changed to work. I have never heard of another airline that operates this way an neither has Jeppesen, their software had no idea how to cope.
Only a management apologist idiot like you would consider an O day a day off.
Go back into your hole.