OD&D defined the RPG genre. It looks like "RPG" became popular as a generic term for games like OD&D, so it's kind of prototypical. If you want history, you can go back as far as you want in Blackmoor, Braunstein and wargames like Kriegsspiel. If you want a more conventionally organized and quickly playable game, you can read B/X. But OD&D is the first full-blown published RPG, the one that inspired and/or provoked the rest.
At the same time, OD&D is not really what we expect RPGs to be. It represents a point in time where games designed on the lines of third edition didn't even exist yet. Now, even when people want to do "old-school," they start with and revert back to standard advice and prevailing styles and whatever they learned RPGs from. They write books full of stuff RPGs are supposed to have. OD&D shows other ways people made RPGs work, other models for gameplay. It provides a fresh angle on the subject, way out of the norm.
I take OD&D as a point of departure, or a point of reference - the RPG equivalent of the biological "model organism" like Mendel's peas, fruit flies, C. elegans, white mice. It's a point in RPG design space, indicating the existence of a larger region of RPG design space, and the presence of viable and interesting designs in that region. And you can study OD&D to see the way this can work, even though it has to work in quite a different way from third edition or whatever.
