Power is a relationship, something that exists between people. Thus, a person can have power advantages and power disadvantages, but not "have power" in itself. The concept of powerlessness should be seen as being severely disadvantaged in the relationship, not as lacking a certain trait. Likewise, a person or structure being said to "have power" should be interpreted as having the power advantage.
When categorism is done from a position of power advantage, it becomes oppression.