Basic Rules

Prime Attributes and Skill Rolls
Open Old School uses the Prime Attributes system from Castles and Crusades.

The Rolls:  Roll 1d20, add any relevant stat bonuses, add any other bonuses that make sense for the roll. Compare to a difficulty. Equal to or over? Success!

Prime Attributes:  Of the six basic stats (Strength, Dexterity, Constitution, Intelligence, Wisdom, Charisma), some are Prime. Characters get two prime stats:  One chosen by the class you take, one chosen on personal preference. Human characters get to choose a third stat to be prime. When rolling skill checks, the base difficulty is 18 for normal stats, and 12 for Prime Stats.

Attribute Bonues

There are six base stats and two secondary stats.

Base:  Strength, Dexterity, Constitution, Intelligence, Wisdom, Charisma.

Secondary:  Perception, Luck.

All Stats have bonuses and penalties on the following scale:

Perception is the one exception, and works on the following scale:

Weapon Levels
Different classes get Weapon Slots, with which they can buy Weapon Levels. These represent someone's proficiency with various weaponry, from being able to hold the sharp end towards the enemy all the way to expert specialization. All weapons start without levels in them, making a character suffer penalties when trying to use them. A spent weapon slot brings a weapon to Level 0, which lets a character use it without penalties. Past that, weapons can be levelled to I, II, III, IV, and V, all of which grant different bonuses to a character when wielding that weapon, depending on the weapon in question.

Rolling Hit Points
Every time a character levels up, they roll their new hit die total in its entirety, adding their Constiution bonus x Level. If the new total HP is larger, they keep it.