Ülevaade
The Old School Renaissance, Old School Revival or OSR is a play style movement in tabletop role-playing games which draws inspiration from the earliest days of tabletop RPGs in the 1970s, especially Dungeons & Dragons.
Prominent standalone rule systems produced within this movement include Old School Essentials, Swords & Wizardry, and Labyrinth Lord.
Description
The OSR movement first developed in the early 2000s, primarily in discussion on internet forums. OSR games encourage a tonal fidelity to early editions of Dungeons & Dragons—less emphasis on predefined endings, and a greater emphasis on player choice determining the fate of characters. OSR Games provide play where wrong decisions can easily become lethal for characters and do not guarantee satisfying endings to character arcs. Characters live and die by player choice as opposed to the story's needs
System Overview & Key Features
Rulings Over Rules
Rather than providing comprehensive mechanics for all potential scenarios, OSR systems rely on the Game Master to adjudicate outcomes based on the players' stated actions. This approach is intended to expedite gameplay and encourage lateral thinking over rule memorization.
High Lethality
Player characters possess low survivability, particularly at early levels. Game mechanics often dictate that a single error or unfavorable dice roll can result in immediate character death. This design element encourages players to avoid direct combat and approach the game world with caution.
Player Skill over Character Skill
Game resolution depends heavily on the player's descriptive input rather than the character's numerical statistics. For example, instead of rolling a die to detect a hidden object, a player must explicitly state that their character is searching a specific area or manipulating a particular mechanism.
Gold as Experience
Character advancement in many OSR games is directly tied to the acquisition of treasure rather than the defeat of enemies. Characters gain experience points primarily by extracting valuables from dangerous locations and returning them to a safe environment. This heavily incentivizes acquiring loot while avoiding unnecessary combat.
Resource Management
Game mechanics require the strict tracking of consumable items such as rations, light sources, ammunition, and carrying capacity (encumbrance). Depleting essential resources in a hazardous environment poses a significant mechanical penalty, making inventory management a central component of gameplay.
Reaction Rolls and Morale
Non-player characters and creatures do not default to immediate hostility. Game Masters utilize randomized reaction tables to determine the initial disposition of encountered entities, which can range from hostile to friendly. Additionally, adversaries are subject to Morale checks during combat, dictating whether they continue fighting or attempt to flee when at a disadvantage.
Additional links
drivethrurpg.com 
https://legacy.drivethrurpg.com/browse.php?filters=45582_0_0_0_0 - DriveThruRPG (Major hub for OSR PDFs and print on demand books)
basicfantasy.org https://basicfantasy.org - Basic Fantasy RPG (A completely free, open source OSR system)