Spoiler alert: NO. OSR is and has always meant TSR D&D Compatible, and 3E isn't that. But I went ahead with the clickbait title. The real question is: Is D&D 3E becoming Old School? And honestly, it may be. There are actually very few who still play this edition. The internet has proclaimed the editions of D&D are AD&D1E, B/X, D&D5E, and that's it (mostly the last one). 3E is chocked in the middle of the fly-over editions. I've heard of 5E beginners getting just fed up enough to walk back to 3.5 to see the "first WotC edition" and get dissuaded by it's intersection of flat math and copious amounts of enumeration. 5E really is a sort of 2nd edition to 3rd edition. In the past I'm sure I've discussed my interest in 3E, which only extends to three settings currently: Scarred Lands, Thieves' World, and Wilderlands of High Fantasy. All of these take different tacts with the 3E ruleset.
Working backwards, because it makes the most sense:
Wilderlands basically pretends disparity in the rules doesn't exist and flatly barely modifies anything with no actually attention given to it. A 4th level character in OD&D Wilderlands is a 4th level character in 3E Wilderlands. This is both good and bad in many respects, but mostly it's just obviously not a 3E setting and can't play as one except under the most stringent of limitations. Limitations it doesn't actually know or apply itself, as what mechanics it does add break the fragile balance the core game possessed at low levels.
Thieves' World is a standard systemic rebuild to meet with the setting's expectations. A throwback to 2E really, dropping most classes (and alignment) in an attempt to get 3E to fit the Thieves' World setting. The difference between this and every 5E setting is that every 5E setting effectively believes everything in 5E is present and usable in the setting in spite of combinations potentially being nonsense - warforged and merfolk existing in Dark Sun, for example. And yes, this will still be a thing come next year's Dark Sun. I guarantee it.
The last is Scarred Lands. A setting that was developed as 3E developed, and built based on all of it's assumptions. Example: You can't really build a 5E Scarred Lands because Warlocks are a thing now, and at the time they were Clerics and Druids, the followers of gods or titans. Building a warlock within these constraints is idiotic because they don't fit the setting. The setting itself is so built out of 3E and procedurally generated, it would take extraordinary effort to explain a new class as broad as the Warlock, when they're functionally identical to Clerics. The organic growth of Scarred Lands makes all the disparate elements of 3E fit together. Even classically difficult classes like Monks and Bards fit the setting well, because the setting was designed intrinsically to fit the ruleset. Down to hundreds of Prestige Classes (which are mostly broken and/or stupid, but they are also present as the system expects).
The reverse engineering in what I'm calling "real time design" was brilliant, far better than Eberron which came later and essentially just looks like D&D + terminators + steampunk elements. While Scarred Lands and Eberron are similar on paper, non-traditional D&D while holding true to the mechanics of the game, There are no reframing of concepts done in Eberron like the new position of Clerics and Druids in the tonality of the setting. It's a "let's add some sprinkles, it'll make it taste different" sort of setting building. I'll probably stop talking about it now,
