He is gaunt and sullen-looking. He is ugly with a squashed nose, a mouth too wide for his face and ears too big that stick out besides. He looks six but claims to be nine. (LoC,Ch5) He is short and skinny with huge ears, nose and mouth. He looks ten. (ACoS,Ch28)
Chronology
Mat finds Olver being accosted by a couple of Mindean men in Maerone. He sends Olver off with Edorion to get a meal and a bath. (LoC,Ch5)
Olver follows the Band of the Red Hand when the head south. Master Burdin feeds him in exchange for helping with the horses. When Mat discovers Olver's presence, he "hires" Olver as his personal messenger. (LoC,Ch22)