The Legal Sets are: Alpha, Beta, Unlimited, Revised, Arabian Nights, Antiquities, Legends, The Dark, Fallen Empires, Collector's Edition, and International Collector's Edition. All reprints of those cards with the original art and original border are also allowed.
Each deck must contain 100 cards, including the commander. The commander must be a legendary creature. The remaining 99 cards must match the color identity of the commander and be singletons (i.e. not more than one of any card).
The following cards are banned in Old School EDH:
The following cards have been errated to be legendary creatures in Old School EDH and may be used as a commander:
Ring of Ma'rûf is legal, but you may only use it to get a card that is legal under the above rules and that you do not already have a copy of in your deck.