The word starting with O that means working extra is overtime. It tends to be a word used when a person works over the expected hours. The pay is typically increased during this time.
obtainable workovertime
phenomenal
Perhaps "phenomenal".
Overtime!
The Latin prefix "Extra" means something that is more than, beyond, or outside of. An example is the word "Extramarital" which means, outside of the marriage.
extra
Quit, retire.Retire means to stop working.
gift
This might be the word "extra." If you have a supply of something, often that means you have extra.
Twice is a word that means two.
Extraterrestrial is a word using the prefix extra- that means outside the world.
spare