Ostler

A horse keeper or handler, someone employed to look after horses. Also hostler.

This was part of a col­lec­tion prints from the 1880s depict­ing let­ters of the alpha­bet, with a pro­fes­sion start­ing with said let­ter. A was for Artist, R is for Rob­ber, X is for xylo­g­ra­ph­er. O, of course, is for Ostler.