From DnD B/X it is a small step to the Original Version of 1974, alternatively called Little Brown Books or colloquially “0e”. The difficulty is that the original 3 booklets have quickly been amended with an assortment of additions and partially reference back to Chainmail. Various Zine articles also clarified ambiguities, added things or changed things, leaving the state of the rules in such a state that it was obviously needed to make a Holmes, later Moldvay, and later Mentzer version to get things back on track and allow normal players, people who did not spend their days hunting for explanations in various places, to play and enjoy the game.
Recently, a variety of enterprising gamers have tried to collect the important bits and pieces and make a version of the “original” rules that can stand alone.
One beautiful and, if people who know a lot about the topic are to believed, particularly faithful interpretation of a particularly good selection of original materials, including Chainmail and including the (optional) Thief class, is “Delving Deeper” by Immersive Ink.
Given the wide range of source material, there are several tidbits that can be argued and come to fisticuffs about, notably the Hit Dice of the Fighter class, but Delving Deeper represents, without doubt, one of the most faithful and most useful “clones” of the “Oe”.
So I tried it, played it in an online campaign, offered to start a campaign to players who were not thrilled about the prospect (it is hard to go from feats and Tiefling Barbarians to basic race as class and d6-HDs) and started to GM another campaign of my own for a group of kids.
But here are the rules for you to come to your own conclusions about their merit: