The name Oliver has another meaning in Norse. It means "Affectionate". This comes from several sources. My suggestion is that if you are naming a child or pet or novel character and the meaning is important, that you research beyond this site, as it somewhat frequently leaves information out.
The defining trait of Oliver in the Song of Roland, is wisdom, of which his name is repeatedly linked to, in contrast to his companion Roland, who represents strength.
Whether or not it originates from "elf army", Oliver surely today is closely related to the olive tree. Olivier is listed as the French version of Oliver, when in fact "olivier" is simply French for "olive tree".