Otis may have fallen in love with Sam Westing's ex-wife Crow—or Berthe Erica Crow, to give her her real name—but that doesn't mean that he can't get upset with her from time to time, especially when winning the game is at stake.
Otis is hopping mad at Crow for losing their clues. In response, Crow tries to reassure Otis that it doesn't matter, as she's remembered all the clues. But that doesn't satisfy Otis at all; he's also remembered the clues, but that's not the point. The point is that someone else could come along and steal their clues, and then where would they be?
As we will discover in due course, Crow hasn't in fact lost any clues; she's given them away. Otis is blissfully aware of this fact, but one can imagine that he would've been even more furious if he'd found out. After all, there he was, partnered with Crow as part of the game, and she was giving their clues away to someone else! It's safe to say that Otis would not have been best pleased.
However, when matters are finally resolved in the end, Otis and Crow are able to put this little argument and all the other ups and downs they've shared behind them and get married.