My Take on Sorcerers

Note: This is mostly referencing BFRPG as it's the OSR ruleset I am most familiar with.

Magic Users study spellcasting for most of their lives, toiling away apprenticing under a wizard for the chance to warp reality with a few simple motions and words, walking away with a single spell and a desire to add to their knowledge.

Sorcerers... Don't. Their souls are warped at birth, eventually developing into innate magical prowess that causes them to be ostracized and shunned by their communities. Thankfully, most low-level adventuring parties are desperate enough to take a Sorcerer into their ranks.

Mechanically, Sorcerers function exactly like Magic Users. They only differ in a few ways...

• Sorcerers do not have spell books, each day they must roll on the spell list for each spell slot. As an example, a fifth-level sorcerer would roll for two 1st-level spells, two 2nd-level spells, and one 3rd-level spell. They may still prepare Read Magic by forgoing a roll on one of their first level spell slots.
• When a sorcerer casts a spell, they roll a d20. The number rolled is the level that the sorcerer casts the spell at, where applicable. If the d20 roll is equal to the level of spell being cast or lower, a wild magic surge happens. Keep a wild magic surge table handy, I personally like this one.
• (Optional) If you really want to make the Cleric question their faith, have the wild surge die decrease a step every time you cast a spell, from d20 to d12 to d10 and so on, resetting once a wild magic surge happens.
• Sorcerers cannot research spells, learn new spells, teach others spells, or create magic items. As their magic is innate and not learned, all forms of magical research (aside from learning about magic) is lost on them. They're less practitioners of magic and moreso fonts of arcane energy leaking into the world.
• Sorcerers can use magic items meant for magic-users. This is probably the only safe way they can cast spells.