Oxygen has 6 valence electrons, hydrogen has just one. Elements want to gain or lose electrons to complete their valence shells of eight electrons. How many atoms of hydrogen does a single atom of oxygen want to bond with? What is the name of this compound?

water

Explanation:

Oxygen has 6 valence electrons, hydrogen has just one. Elements want to gain or lose electrons to complete their valence shells of eight electrons.

When oxygen combine with hydrogen it needed two electrons to complete the octet that's why two hydrogen atoms are combine with one atom of oxygen.

2H₂ + O₂ → 2H₂O

Both share the electron and form covalent bond because electrons are not equally share.

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial positive.