It's a sci-fi story where this guy travels to different worlds and saves them. All I specifically remember is that on each world he goes to, the people had differently shaped ears; I think in the last book I read, they had square ears. By the fourth book, the story was getting a bit redundant, but I still wanted to find out how things turned out.

I remember the title of the last book I read was 'BLANK's Opal' (just gotta fill in the blank). It was a name that began with an O. Anyone know what I'm referring to?