Most OSR games, in particular retro-clones, have a tendency to limit clerics and other divine casters from acquiring high-level spells. This is usually because they're based on either AD&D, in which divine spells only went to level 7, or B/X, in which all spellcasters are capped off at 6th level spells. I thought it would be interesting to come up with a way to allow divine casters to gain spells beyond that limit through interactions with the campaign world.
I think the best way for divine spellcasters specifically would be for them to make some kind of deal with their diety. Similar to magic-users patrons in games like DCC, the cleric could call upon whichever god they serve and take on an oath in exchange for more extraordinary powers. These oaths could be anything from an oath of poverty to a promise to complete a divine quest. Should the cleric break this oath they should at the very least lose access to their higher level spellcasting and may incur other punishments from their diety. This can lead to interesting campaign hooks while also giving your high-level clerics a more direct relationship with their diety.
The exact powers the cleric receives are determined by the referee. It is probably easiest to give them a single spell slot per oath and use a spell list from a version of the game with higher-level divine casters. But it could also be interesting to make their new spells into class abilities like turn undead or have the PC make oaths for knowledge of spells rather than for spell slots. Either way, the reward for an oath should generally be an equal reward to the severity of the oath. A character who swears to never lie to a non-chaotic creature should not receive the same reward as a character who swears an oath of eternal silence. The cleric and god should negotiate as to what the cleric receives in exchange for their oath, but keep in mind that the diety holds all the cards.
Though this is mainly meant for clerics, oaths could also be interesting for paladins and druids. Paladins' high combat ability is balanced by their fairly limited spellcasting, so allowing them to make an oath for more power could lead to an immensely powerful character. Because of this, their oaths should be especially restrictive. Druids typically aren't depicted as having patron deities, but they might make deals with animistic spirits or some sort of fey council. the oaths of druids and paladins should be different in nature from those of clerics, and clerics should have different oaths depending on what sort of god they worship. A god of torture will require different oaths from their followers than a god of kindness.
Oaths can also be used in other situations. A non-divine caster may take one up out of religious devotion, or in exchange for some sort of divine intervention. An enforced oath of poverty or silence might also make a good punishment for a misbehaving cleric. A fighter might take on an oath in exchange for a magic weapon, losing that weapon if they break their vow. There are plenty of potentially entertaining ways to build off oaths as a concept.
The following is a d12 table of potential oaths. They are of varying severity in order to show how wide a gap there can be in the limitations of an oath. One can imagine a god of fishermen giving their cleric a 7th level spell after they take on an oath of poverty, or a god of chaos granting their servant level 9 spells so that they can fulfill their crusade oath, leading an army to destroy all lawful settlements they encounter. I tried to keep the descriptions fairly vague so that the referee can determine the specifics of the oath.
