Most elements of the OSR playstyle– sandbox campaign design, player skill, exploration focus– are centered around maximizing player agency. In general, the more agency the players have, the better your campaign will be.
Player agency is generally a rather nebulous concept, more of a feeling than something that can be clearly defined and measured. Until now, that is. I've come up with a (rough but serviceable) measurement for it.
I call it TFMC: Time to First Meaningful Choice.
In short: how long into a session is it before the party gets their first opportunity to make a meaningful choice?
"Do we explore the megadungeon some more or go track down the fugitive for the bounty" is a meaningful choice. "Do we hire some retainers before we go back into the dungeon" is a meaningful choice.
"What games do we play and what food do we eat at the harvest festival" is probably not a meaningful choice, nor is "what route do we take to the adventure the DM has decided we're going on." Choices will real consequences that have to be made completely blind also aren't meaningful, like if one of those two routes is safer but the more dangerous one offers a side quest, but you have no way of knowing that before you pick your route– not a meaningful choice.
In other words, a meaningful choice is a) consequential, and b) made with enough information (or the opportunity to gather information, even if you failed to do so) that the party can actually make an informed choice.
A few clarifications: purely out of character stuff like recapping the last session or helping people create characters doesn't count towards session time for this purpose. Introducing new characters does count, as does narrating any in-universe events that happen between sessions, like if you have a random events table you're rolling on. In other words, only time spent actually playing counts.
Measure this TFMC every session. After ten sessions or so, take an average for your campaign.
I've been in 5E games where the TFMC was two hours or more. A couple sessions lasted 4+ hours and had literally no meaningful choices. That's not a game; that's an extended cutscene.
A good OSR campaign should almost always have a TFMC of under 5 minutes, and pretty much never over 15 minutes. The average should be under 10 minutes.
Even that five minute target is only due to the aforementioned qualifiers– sessions may begin with in-character character intros and the referee narrating an event that happened. Or you may enjoy starting sessions with a few minutes of in-character shooting the shit that doesn't affect the plot; nothing wrong with that if the whole group's into it.
One obvious objection to this metric is that you could have one meaningful choice early in a session, and every few thereafter. Which, sure– it's a simplified metric. In practice though, railroad-y campaigns generally have sessions structured so that the pre-plotted stuff is first and the part where the players make meaningful decisions is in the later part of the session.
You can probably see why– if this were reversed, with the meaningful choices first and the railroad stuff second, the choices made by the party could derail the session. Or to avoid that, the DM would have to negate the player's choices in ways that would be unsatisfying even by the standards of people who otherwise like pre-plotted campaigns.
I've had maybe two or three sessions like that, ever. And also maybe three or four sessions that were the reverse, with high player agency but the first meaningful choice was over 15 minutes in.
So in practice, I find this TFMC metric tracks very closely with the overall sandbox-y-ness and player freedom of a campaign.
And just as important, as an OSR player I find it tracks very closely with how much I personally enjoy a campaign; when I do play non-OSR games, I find this metric more than anything else tells me early on whether I'm going to enjoy the campaign, or should make an excuse to gracefully drop out of it.