I'll keep updated this going forward as there is a ton of notable minutia.
- Fighters get 1 extra weapon proficiency.
- Fighters are presented with the UA optional rule of Specialization
- Fighters 'sweep' is spelled out in OSRIC. It exists in AD&D but it's hidden under the attacks/round section after the ranger entry.
- Only Fighters Sweep in OSRIC, it's likely that all Fighter Sub-classes sweep in AD&D. (Fighters vs fighters notation)
- Classes have meaningless extra minimum attributes (ex. AD&D Clerics just need 9 Wis, OSRIC Clerics need Str 6, Dex 3, Con 6, Int 6, Wis 9, Cha 6)
- Differences in extreme ability scores detail (out of character generation range).
- Differences in ability score headers (minor and major tests of strength)
- Half-elves cannot be druids.
- Elves, Gnomes and Dwarves could only be NPC clerics in AD&D, but now can be PCs. Half-orcs are capped at 15 for assassin and 7 for thieves.
- Assassins get -3 non-proficiency with weapons instead of -2
- No monk, psionics, or bard classes.
- Certain details of the weapons table (ex. crossbows do more damage and "pole arm").
- No weapons vs. ac type.
- No weapon speed factor.
- Some spells have been renamed.
- Simpler unarmed combat. No rules for grappling, pummeling, etc
- Some creatures and items have been renamed or removed ( ex. No Mind Flayer or Eye of Vecna).
- Details of the thief's skills progression are different.
- Rules for sickness and disease are greatly simplified.
- Experience tables differ slightly. (ex. A fighter only needs 1900 XP to get to level 2)
- Codification of the 'spill over' segment management system (ex.You go on segment 6 but are casting a spell with the casting time of 5, therefore your spell goes off segment 1 of the following round).
- Codification of the surprise segment range (ex. monsters roll a 5, players roll a 2, the party is surprised for 2 segments - Monsters surprising on a 1-3 can surprise up to 3 segments.)
- Ranged weapons 'ranges' are presented increments rather than literal thresholds, but are mostly the same thing.
- Damage in OSRIC taken by an unconscious character aside from bleeding is instantly killed.
- No apparent encounter distance specifications in OSRIC that exist in AD&D.
- Hold person clearly affects certain things in the PHB of AD&D, ambiguities are in the DMG, OSRIC leaves it mostly up to the DM.
- OSRIC does not list item XP values
- Turn Undead in AD&D affect 1d12, in OSRIC it's 2d6
- Turn Undead thresholds are different
- Many Magical Items have small or significant changes.
- There are OSRIC Magical Items that do not exist in the DMG or UA
- It is specified in OSRIC that only single-classed characters can gain experience bonuses for high ability scores.
A lot of minutia that is likely not intended may be found here:
https://knights-n-knaves.com/phpbb3/viewtopic.php?f=45&t=10353
