Acids and metals react to produce hydrogen gas, not oxygen. The general reaction is:

Metal + Acid → Salt + Hydrogen gas

For example, when hydrochloric acid (HCl) reacts with iron (Fe), it produces iron(II) chloride (FeCl2) and hydrogen gas (H2):

Fe + 2HCl → FeCl2 + H2

The hydrogen gas is produced because the metal atoms lose electrons to the hydrogen ions in the acid, forming hydrogen atoms which then combine to form hydrogen gas molecules.