Check out the heart of Codex Martialis – the combat system which can be layered over other RPGs, and was specifically designed to be compatible with 3.5 OGL.
The Core Rules include the basic fighting mechanics: Martial Pool, Three Range Bands, Binding, Grappling and Dynamic Critical Hits. It includes a long list of Martial Feats, special fighting abilities which enhance the luck and fighting effectiveness of your character. It also includes detailed lists of weapons, shields, and armor. There is a section with details on different types of wild animals and how they fight, sample characters, and several optional rules including a Detailed Wound System.
Codex Core rules only cover combat and do not include any rules for character creation, magic or running a campaign.
Codex Martialis Core rules were originally released in 2008 (Version 1), revised between 2010 and 2012 (Version 2-2.5) and revised again in 2020(Version 3.2).