Clerical NatureTLDR The White Box Cleric makes a great template for custom OSR character classes.
There are three human classes detailed in the three little brown books. They cover a range from pure combat specialist (the fighting-man), pure magic (the magic-user), and a hybrid of the two (the cleric). That last one, the cleric, being part arcane, part martial, with a special ability mechanic (turn undead) can be a valuable template for designing our own original D&D classes.
As was pointed out in this forum discussion, the turn undead table works like the Monster Reactions table: a 2d6 toss determines whether or not the turn attempt is successful and the effectiveness of that success: either a simple turn of the creature or its destruction. A Generalized version of that table might look something like this:
The table could be generalized even further: the ability can only affect an opposition less than two levels above the level of the user with a target number of 7, modified by +2 for every level the obstacle has above the user and -2 for every level the user surpasses the opposition; final target numbers below three are considered automatic, those above 11 are considered impossible.
The opposition level could represent the HD of a creature or creatures, the level of a dungeon, the level of a spell or spellcaster, or really any category of strength or power that you’d like to imagine. The result on the table can be read as a simple pass/fail binary or with varying levels of success and failure, such as, for example: meet target number for a success with cost; beat target by 2 for a success with no cost; beat target by 4 for a success with advantage; fail target by 2 for a failure with cost; fail target by 4 or more for a catastrophic failure / failure with disadvantage.
For templating purposes, this table can become the basis for any number of class abilities that we want to both scale up with level and challenge with increasing or varied difficulties. Depending on the nature of the abilities, that might be all that’s needed. Turn Undead, for example, though powerful, is situational— only working against undead type creatures of a maximum level approximate to the level of the cleric, and therefore it makes sense to supplement it with a type of spellcasting or other ability. A thief, on the other hand, has abilities that are very generally useful and therefore needs no or little supplementation. Martial abilities can follow the cleric’s example or be adjusted as necessary to achieve the class balance that you’re looking for. Additionally, restrictions can be added to further balance the class.
Let me know on Twitter if you find this useful (or not) or if you have any suggestions or ideas.