The oxygen of the hydronium ion

The oxygen of the hydronium ion has approximately tetrahedral geometry and is therefore sp^3-hybridized. Two of the sp³ hybrid orbitals contain an unshared electron pair. One of these becomes the lone pair in H3O⁺; the other overlaps with a proton (H⁺) to give one of the O-H bonds. The other two sp³ hybrid orbitals contain one electron each; each of these overlaps with the 1s orbital of a hydrogen atom (which contains one electron) to give the three sp³-1s  bonds (the O-H bonds) of H3O⁺.