What follows is a basic summary of how OSE is generally played. I will not get into the minutia of individual terms, tables or stats for the most part, but I will give the broad strokes to help get you started! For more information, please refer to the basic rules pdf, or the full rules found in the google drive
Ability Scores: The basic abilities of your character. STR, INT, WIS, DEX, CON, CHA. Unlike in other games, these numbers are not often rolled on to complete tasks. When they are though, a d20 is rolled and a success happens when you roll bellow your stat.
Prime Requisite: This is one of your abilities scores that is defined by your class as being important, having a high prime requisite allows you to gain more xp
Alignment: In OSE there are only three alignments: lawful, neutral and chaotic
Armor Class (AC): AC is the amount of armor your character has protecting them. We will be using ascending AC which means that the bigger your AC is, the better it is.
Attack bonus: This is your general bonus that you add whenever you are rolling to hit something. Since we're using ascending armor class, we are not using THAC0.
Saving Throw Values: These are your ability to avoid certain dangerous effects. When rolling a saving throw, roll a d20 and succeed on a roll higher than your Save Value
Movement Speed: Movement speed is split up into three categories: Base, Encounter and Overland.
Health and Healing: When you adventure you will probably lose some health along the way. Unlike in fifth edition, healing takes one full day of rest in civilization, and for each day you do so, you regain 1d3 hp. If you are reduced to 0 hit points, you instantly die.
Time: In OSE, time is broken up into three different measures. Rounds, Turns, and Watches. A round is a period of 10 seconds of game time and is the way time is kept during combat. A turn is a period of 10 minutes game time, and is kept during dungeon exploration. There are four watches in a day, and watches are used to keep track of time during overland exploration.
Dungeon Adventuring: When in a dungeon, during every turn, the players move cautiously through the dungeon and investigate strange happenings. Map making is encouraged in a dungeon in order to know the way back out or so you can revisit the same locations.
Experience and Advancement: Some experience is gained when killing monsters, but most xp is gained through treasure taken back to civilization. For every gp worth of treasure that you are able to salvage (not necessarily spend or sell, only what you carry out) you gain 1 xp.
Combat Sequence: The general sequence of combat is this each round: Certain actions (spells and melee movement) are declared, Initiative is rolled, winning side acts followed by losing side.
Rolling to Hit: When rolling to hit an attack, you roll a d20 and add your attack modifier, this will be based on your level, and, depending on what kind of attack you're making, DEX or STR.
Damage: When rolling damage, STR is added to melee attacks, but DEX is not added to ranged damage. Damage is based on the weapon being used.
Spell Slots: The number of spell slots you have determines the number of spells you can prepare each day. You can prepare one spell into each slot, so you have to choose exactly which spells you take with you.
Arcane vs Divine spell casting: While both Arcane and Divine spell casters use the same system, they have slightly different restrictions. Arcane spell casters prepare spells from a spell book which can only be expanded by finding new spells, however they are able to create new spells. On the other hand, Divine spell casters pick from all available spells each time they choose spells, however, they are bound by their deity.
Spell Books: For arcane casters, their spells are held in a spell book, new spells from other magic user's books may be copied at great cost.
Deity Disfavor: For divine casters, if they use their spells in a way that would go against their deity, they risk losing their favor and therefore losing their magical abilities.
These are really just a short summary of all the rules for this game, please at least read the OSE basic rules pdf linked here for a slightly more comprehensive idea of the rules