[adv] from on board a vessel into the water; "they dropped their garbage overboard"
[adv] to extremes; "he went overboard to please his in-laws"
OVERBOARD is a 9 letter word that starts with O.
Webster's 1913 Dictionary
Definition:
\O"ver*board`\, adv.
Over the side of a ship; hence, from on board of a ship, into
the water; as, to fall overboard.
{To throw overboard}, to discard; to abandon, as a dependent
or friend.
WordNet Dictionary