Here's how I'm getting rid of Clerics and Druids and Paladins in my OSE based game:
The classes are unavailable for both PCs and Players. Their spells - with the exception of Cure Light Wounds, Cure Serious Wounds, Commune, and Raise Dead - are available to Magic-Users.
Randomly generated Magic-User spells on level-up are still generated from the Magic-User Spell list, so the spells have to be obtained through play.
Where a Druid or Cleric spell also appears on the Magic-User list but at a different level, use the spell level from the Magic-User list.
|The Cleric class illustration from the Mentzer Basic Set|
As for how it'll impact the game, I've added a Bonus Hit Point mechanic which I think will more or less balance out the lack of Cure Wounds spells.
On the DM side - where a pre-published module features a Cleric I will either convert them to a Magic-User, to some sort of Cultist, or a Fighter with religious inclinations (which ultimately is just another flavour of Cultist) as appropriate.
|The Cleric class illustration from the Mentzer Expert set.|