One of the simplistic and fun parts of OD&D and the Swords & Wizardry retro-clone (pre-Black Blade Publishing's involvement*) is the exclusion of the Thief class. The early game assumed that all adventurers could do actions not defined by their class. The inclusion of the Thief later on slanted the game in perception if not mechanically that you need a Thief to do things that were the province of the common adventurer before.
The World of Onn is a Thief-free game that uses a simple skill system that builds from the early editions of OD&D and B/X D&D that uses the 33% (or 2-in-6) chance that a given action will succeed if a die roll is called for. The core mechanic for Onn's Skill System is the use of d12's for skill attempts that require a die roll. The base chance of success is a roll of 1-4 (the same 3% chance as 2-in-6), with the chance of success modified by the character's relevant ability score adjustment. So a character of 16 Dexterity (+1 adjustment) would have a chance of success of 1-5 on a d12 to pick a standard lock, while an 8 Strength character would have a 1-3 chance of success on a d12 to climb a wall.
Optional Skill Rules for Onn (and Swords & Wizardry)
No character is restricted from performing any activity. Each adventurer can try to sneak up on their enemies, pick the lock on a chest that hopefully contains treasure and magic, jump across a wide pit in their way and the like. As an optional set of rules additions, characters can acquire skills that increase their chance at performing various activities. While some characters will find some skills mesh well with their chosen class, no skill is barred from a character based on his class. Characters begin play with 4 Skill Slots. A character’s Intelligence adjustment also affects the starting number of Skill Slots, adjusting the total number from 2-6 (3-5 if using S&W's -/+1 limit for ability score adjustments). Characters gain a new Skill Slot at 4th level and every 4 levels beyond (8th, 12th, etc).
At 1st level, characters must use each slot for a different skill. After 1st level, when a character gains additional slots, he may use them as he sees fit. For example, if a character decides he wants to be better at sneaking he may dedicate 1 slot to the Sneaking skill at 1st level and then dedicate another slot at any level he gains a new slot after 1st level.
When required to make a skill check (if success is not guaranteed), the procedure is the same as using ‘Dungeoneering Skills’, 1-4 chance of success on a d12 modified by the relevant Ability Score adjustment, with the following modifier - for each slot dedicated to a skill the character’s chance of success increases by 1. For example, if a character with a 13 Dexterity (+1 adjustment) and 2 Skill Slots in Lockpicking (+2 bonus), would have a base chance of success of 1-7 on a d12 to pick standard locks. The Referee may assess a modifier to the chance of success based on the conditions the skill is being attempted in. The amount of time required to perform a skill must also be determined by the Referee - picking a lock could take 1 turn or less (or 1d10 rounds), but a Decipher Writing check to translate an ancient scroll could take an entire day.
You can help another character achieve success on some skill checks by making the check in a cooperative effort. In many cases, another character’s help won’t be beneficial, or only a limited number of characters can help at once. Generally the chance of success should gain a +1 bonus if the help of additional characters is relevant.
The list below is a sample list of skills. Referees are free to add or remove any skills they wish, and to define the skills how they are to be applied in their individual campaigns.
SKILL - RELEVANT ABILITY
Appraise Item Intelligence
Decipher Writing Intelligence
Disable Trap Intelligence
Escape Artist Dexterity
Healing (First Aid) Wisdom
Opening Locks Dexterity
Read/Write (language) Intelligence
Ritual Casting Constitution
Speak Language (language) Intelligence
Wilderness Survival Wisdom
*I mention this because one of the conditions Black Blade made to produce the book for brick and mortal sales was to provide more 'meat' in the Swords & Wizardry Core Rules book. In order to do this, some optional classes were added, including Thieves, among other things. Even though I'm not personally an advocate of the Thief class, I don't disparage anyone else's game for including them. This isn't a negative thing in my mind, just a fact of record.