I've been working on a sandbox campaign for the last weeks, and recently started to wonder why I wanted to run it in 5th edition in the first place. OSE just seems so much neater.
I think a big advantage with OSE (and of course B/X) is that it's lightweigt nature makes it particularly well suited for improvisation and procedurally generated situations. When you realize you need an encounter with a certain type of monster right now, you can put it together in a minute. No need to go through a whole monster description to check what abilities it has and how they work, and how that all translates into useful tactics. If some kind of challenge comes up that you had not thought of before, there's not much in the way of special rules that you could look up. You could of course just make something up on the spot with other editions as well, but I always find it really annoying when I know a mechnic for this situation exists but I would not use it because I'd have to look it up. Making something up that's not covered by the rules feels very different than making something up for something that already has a default rule.
I feel OSE is so simple that when the players have a random encounter with goblins and decide to track a fleeing goblin back to its lair, I could put that lair and the rest of the goblin tribe together while the players are still talking and have it ready by the time they enter the cave entrance in the forest.
Even with the simplicity of 5th edition, I think I would have to take a 15 minute break for that. Which isn't so great if that's the kind of thing that happens once or twice every time you play.
The super simple rules open up new possibilities for running a campaign.
