Question
My oak tree has yellow and red small bumps on it's leaves
Some on both sides. Is this normal or is it a fungus?

Answer
They are called galls caused by an insect (small wasp) The wasp lays its eggs on the surface of the leaf or twig and secretes a chemical that causes the tissue of the leaf or twig to swell and cover the egg. the egg hatches and moves on. The galls themselves will not harm the overall health of the tree although they can look bad. Leaf galls may cause the leaves to drop prematurely. There are many types of these wasps some attack the twigs and others the leaves. Control is not usually needed and has to be timed with the wasp presence and not the galls themselves. Here are a web links to a couple of sites on gall wasp that discuss the wasps and control measures.
http://entomology.unl.edu/ornamentals/pestprofiles/gallwasps.htm
http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note05/note05.html

Not a real health problem to the tree. I would not worry about these galls.