I think people are underestimating the role of proper crowd control in all of this. From what I have read online and experienced myself on Canada Day and during Bluesfest, crowd control has been an absolute gongshow.
Specifically on Canada Day, there was unexpected fencing everywhere blocking peoples' intended exit routes, inadequate signage notifying people of detours they must take, with said detours leading people on long, roundabout, unnatural patterns, it was horrible. And worst of all, there had, and continues to be, not been nearly enough staff directing crowds on where to go, and the staff they do have are horribly misinformed and lack a cohesive message. Lots of mixed messaging causing people to backtrack, which is what has been causing most of the chaos. Not lack of capacity on the O-Train.
The whole time I was in that mess, I couldn't help but contrast it with my experience living in Montreal and attending the F1 Grand Prix and Osheaga many times. The GP is held on Ile-Notre-Dame, adjacent to Jean Drapeau Park. Pretty much the only access to the site is via Jean Drapeau metro station. The GP attracts 300K+ people over the three days and almost all of them are trying to leave at once after the final session each day. What's worse, is that they are almost all trying to use the same platform at the station, since the other direction heads toward Longueuil. Of course, the metro has a much higher capacity than the O-Train, but still not enough to handle that many more people.
So what's the solution? The army of STM/city police officers who direct crowds loudly, clearly, and cohesively. Leaving the site, you approach Jean Drapeau station and take your place in a massive lineup outside the station, which appears chaotic at first but you soon realize it's quite orderly. Every few minutes, you move forward a few steps, as another trainload of people is cleared away. All the while, you have cops yelling assertively, yet respectfully, to encourage people to keep moving whenever possible and to have your fare ready as you approach the gates. As you pass the fare gates and inch forward and down towards platform level, you encounter dozens of cops forming a literal human barrier when the platform has reached capacity to avoid a crush situation where people are pushed onto the tracks. When a train departs and the platform is cleared. The cops yield to the next batch, who are promptly encouraged to go as far down the platform as possible, and to use the full depth of the platform. I cannot emphasize enough how intently they encourage you to use all available space on the platform - every inch of the platform is used, but you are never so tight that you are pushed around thanks to the well-controlled platform access. Once you get on the train, you feel like you have spent an hour+ waiting since the time you arrived at the station. However, you check your watch and realize it's only been 20-25 minutes, which is more than reasonable for a crowd of that magnitude.
Of course, you also need cooperation from the public to make it happen so seamlessly. It'd be easy to see the massive line as you approach the station and try and filter your way to the front, leading to pushing/shoving. But the main reason people remain patient is because they know that, even though the process feels long, they will get through in a timely manner (and because there are cops everywhere watching the crowd like hawks, discouraging disorder).
The above is EXACTLY what Ottawa needs to learn how to do during big events like Canada Day or big acts at Bluesfest. No haphazard adjustments, no sending people in the opposite direction to make them someone else's problem, etc. Just a clear, well thought-out strategy that is repeated until the whole crowd is fed through.
As far as events at the future Lebreton arena, we just need a scaled back, yet equally definitive strategy to manage crowds in the 17-18K range, and I think the O-Train will be able to manage just fine that way.