The oxygen side of a water molecule is
neutral
slightly negative
slightly positive

Answer is "slightly negative".

Explanation

In water molecule, there are 3 atoms which belonged to 2 elements. They are 2 H atoms and 1 O atom. Oxygen atom is more electronegative than H atom. Hence, Oxygen atom in the water molecule gets a slightly negative charge on it.

What is electronegativity?

Electronegativity is a value that tells us how an atom can attract electrons towards itself.
It helps us to tell the nature of molecule (as polar or non-polar) and the nature of the bond between atoms (as ionic or covalent). 
If the electronegativity is high, then the attraction to the electrons is high unless the attraction to the electrons is low. 
If the attraction towards electrons is high, then that atom gains a negative charge on it.