The term "on brain" can refer to a variety of mental states or preoccupations. Synonyms for this phrase might include "in mind," "top of mind," "obsessed with," or "focused on." Depending on the context, other synonyms could include "preoccupied," "burdened with," "plagued by," or simply "thinking about." Of course, the exact synonym that best fits a particular situation will depend on the nuances of the moment and the specific connotations of each word. However, these synonyms provide some useful options for anyone looking to express themselves more effectively in writing or conversation.