Oxygen is very reactive and can react with the majority of other elements; also oxygen react with all organic compounds etc.
Many chemical elements combine with oxygen; the reaction with oxygen is called oxydation.
Oxides are formed when nonmetals combine with oxygen.
When hydrogen and oxygen combine to form water, water is a product of the reaction.
When you combine one Oxygen and one Hydrogen you get Hydroxide. If you combine one Oxygen with two hydrogens you will end up with water.
they make water
When oxygen and hydrogen combine, they form water.
Covalent
Yes, and often does. For example, the DNA in your body is impossible without it.
minerals
Yes
water
we can combine any other element or even an compound witw it like hydrogen+oxygen=water