The word "oner" is a fairly uncommon term that means a difficult or burdensome task. There are several synonyms that can be used in place of oner to convey a similar meaning. One of these words is "onerous," which means something that is particularly difficult or laborious. Another word that can be used in place of oner is "arduous," which means something that requires a great deal of effort and persistence. "Burden," "encumbrance," and "hindrance" are also synonyms for oner that imply something that is challenging or difficult to deal with. When referring to a specific task, "challenge" or "obstacle" could also be used as synonyms for oner.