One electron or can more than one be share in covalent bonding?

When an atom shares an electron to fill its outermost shell is it only one Electron be shared, which would lead it to seek another bond with another atom if it needs more that one elctron. Or can it share more than one electron with another atom. For Example, Oxygen: has to form O3 because it has six valenece electrons and can only share one with the other two Oxygen atoms to complete 8. Or can it just be O2 and share two electrons with the other to complete the 8 nessary.