Meruem and Oga are both kings and as such have to try to rule in a way that would best serve their kingdoms and ideals. Oga is quick to abandon those ideals of his though which is why he will never be a match for Meruem. Meruem also has a great deal more power which is another thing to consider. It would be difficult for Oga to react to Meruem’s attacks, let alone counter them. Meruem wins.