No, I don't think the OD&D rules are available as legal PDFs, which seems really odd since the supplements (Greyhawk, Blackmoor, etc) are available for purchase. I don't know why WotC hasn't released the OD&D rules as PDF.
You can get them on ebay for $30-$50 dollars, if they're not mint or one of the rarer printings.
Another option would be to go with the Holmes "blue book." It's much closer to OD&D than to later versions of Classic D&D (Moldvay/Cook/Marsh or Mentzer). The main drawback is that it only covers levels 1-3. Still, with the Holmes book and the OD&D supplements you could run a decent game.