I think it's pretty rare that OSR games need conversions. Everything I'm familiar with except for Shadowdark and Mork Borg can run as-is, and even those just need a little bit of practice to be able to do it on the fly.
Shadowdark wants monsters to have the six ability scores, which can be handled by giving them all a flat 10 except when the critter would be more potent, and the DM can just give them a 16 at that point.
And Mork Borg needs a morale stat (which many OSR games include already) and possibly an armor die, which can probably be determined, most of the time, from a monster's armor class.
But for games that are more directly built off of TSR D&D -- the vast bulk of OSR games, in other words -- at most, one just has to flip ascending AC to descending AC, depending on which ruleset something is being published for or used.