The terms "old girlfriend/boyfriend" and "ex-girlfriend/boyfriend" are often used to refer to past romantic partners, but there is a subtle difference in their connotations:
Old girlfriend/boyfriend: This term simply refers to someone with whom you had a romantic relationship in the past. It could be a former girlfriend or boyfriend from any point in your life, regardless of whether the relationship ended recently or a long time ago. The term "old" here doesn't necessarily imply that the relationship was significant or serious; it merely indicates that the person was a romantic partner at some point in the past.
Ex-girlfriend/boyfriend: The term "ex" is short for "ex-partner" or "former partner." It specifically denotes that the romantic relationship has ended, and the person was your girlfriend or boyfriend in the past. The use of "ex" implies that there was a more defined and recognizable relationship that has since come to an end.
In summary, the difference lies in the specificity of the term "ex-girlfriend/boyfriend," which emphasizes that the relationship has officially concluded. On the other hand, "old girlfriend/boyfriend" is a more general term for any person with whom you were in a romantic relationship in the past, regardless of whether the relationship is ongoing or has ended.