I just assumed that he told them what would happen in the final volume.
To be honest, I'm not sure if O'Malley himself knew what was going to be in the final volume that far back, beyond perhaps the vaguest of outlines.
I understand that prep for this movie started while Vol. 3 was still in development. To that end, I think that we can expect the first two volumes to carry over with extreme fidelity (the trailer has already proven that), but the two stories will branch out increasingly far after that point.
For instance, note that Roxanne attacked with that strange prehensile whip, though she stuck with a plain sword in the graphic novel. Additionally, the Katayanagi twins attacked with robots in the GN, but they seem to appear in the trailers as a DJ duo. From this, we can suspect that while O'Malley had the names and appearances of the seven exes planned out, he clearly didn't know as much about their attack methods at the time of production.
Then there's Envy Adams. If you follow O'Malley's blog or read his "Annotated Scott Pilgrim" series, you'll know that it took a long time for O'Malley to get a handle on that girl. She went through all sorts of background changes and appearance makeovers behind the scenes before she finally appeared in Vol. 3. It's possible that O'Malley might have established her character in collaboration with Edgar Wright while the movie was in development, but I wouldn't be surprised if the movie's Envy is noticeably different from the GN's Envy.
Also, if Knives' dad or Lisa Miller have made the cut, I'll be very surprised.