If we do Mg + O2 where O2 means Oxygen not Oxide then why do we use the valency 2- which is for Oxide?

single oxygen atom is having capability of accepting 2 electrons to get reduced and single magnesium atom is having capability of donating 2 electrons. but in nature when we burn magnesium strip the oxygen available for the reaction is O2 and not single oxygen atom. so its 2Mg + O2 gives 2MgO. clear your funda about valance. its capabilities of losing and gaining electrons only which makes bond between two atoms.