what i see from the story is finally they are staying together. but not by fitting the smaller piece into the big one, but by rolling together, which means accompanying each other in the life.
Wouldn't it be a lovely ending if the missing piece stay forever with the right one it first met? I mean, the one they eventually have to separate because the missing piece is growing (first love), not the Big O...
It's nice that in the end the missing piece can roll along with the Big O, but that's not the missing piece anymore...