I don't recall anything in the OD&D books that mentions what happens when you gain enough experience to increase two or more levels, instead of just one. I know in AD&D, you only get enough XP to increase one level, plus one point shy of the next level; anything above that is excess XP and is lost. I referred to the AD&D version when I came up with the house rule that level drain does not decrease experience points, and you can regain one lost level by going on an adventure and earning at least one point, which assumes that the "you only gain one level per adventure" rule is in effect.
But now I'm thinking: why not expand that rule? You still only earn one level at most per adventure. But forget the bit about losing excess XP, You earn what you earn, and your XP never decreases. If a 1st level thief somehow earns 5,000 XP, the thief goes up to 2nd level, but after two more adventures worth at least 1 XP, the thief will be 4th level.