Why is the first ionization energy always greater than the second

The amount of energy required to remove electron from the outer most shell of neutral isolated gaseous element to form unipositive ion is known as first ionisation potential.

The amount of energy required to required to remove electron from unipositive ion to form dipositive ion is known as second ionisation energy.

As the positive charge increases size decreases but nuclear charge increases. Due more nuclear charge in unipositive ion compare with neutral element, second ionisation energy value is more compare with first ionisation energy.

                   hope it helps u

if you mean hydrogen and oxygen then water is the product