onlay and inlay are used as restorations and are meant to replace cusps on teeth, the cusps are the high point on your teeth that you can feel with your tongue....
veneers and onlays are not the same thing....if you get veneers they are going to shave down your teeth a little and cement the veneers to them....
try bleaching and then ask about veneers.....
Its always a better idea to keep your natural teeth. If you can afford it, I would probably get several opinions and try to do the bleaching first. If that doesnt help, THEN and only then would i get veneers (what he probably is calling onlays).