While the OSR is full of more systems than you can shake a bloody, battered stick at, there are things that can be used with most any OSR system. Magic items, monsters, and therefore adventures can all be used in practically any system regardless of their origin. However, these same items will not transfer nearly as smoothly to non-OSR systems (Fate, 5e, PF, etc.) due to several factors. Because of this, I "propose" the term OSR-neutral to mean anything that can be used with any system based on TSR-era D&D. Note that while I do think Traveler, Tunnels and Trolls, and similar games can be considered old school, and their retroclones therefore OSR material, the bulk of OSR material is based on D&D. A good example of OSR-neutral would be me running Monolith from Beyond Space and Time, a LotFP module, with Swords and Wizardry Whitebox.
Also note that I don't mean genre hacks'. such as the Rad Hack or Operation Whitebox, material could be used readily in Lamentations or Labyrinth Lord. That obviously wouldn't work without due consideration due to the different intent involved in the creation, at least in my opinion.