The antonym for "being one back" could have various meanings depending on the context. In a sports competition, the antonym would be "ahead" or "in the lead." In a business context, the antonym would be "in front" or "winning." The opposite of "being one back" could also be "catching up" or "closing the gap" in a race or competition. In a social context, the antonym of "being one step behind" could be "on par" or "equal" to indicate that one is not lagging in social status or reputation. In general, the antonym of "being behind" is "being ahead" or "being equal," depending on the situation.