Open Core is based on the OGL but differs quite a bit from it. It is designed to allow the GM to create his/her own game with the tools provided. It uses basic roll system; 3d6 + Attribute + Skill. (or even 1d20 + Attribute + skill). The system is modular, so you can take or leave what you like.
Like a number of games, OC uses a point buy system for attributes and skills. The number of points granted is based on the game's power level. You have six attributes, three physical and three mental, that map nicely to d20 or even Unisystem. These abilities then can be used for derived abilities, like initiative or health or even variant ones like SAN.
Skills are covered and use the same point buy system. And there are a lot of skills, though given the modular nature of the game and maybe the your own games focus, you might need to use all of them.
There are also a number of effects-based abilities which look very similar to BESM d20/SAS d20 or other open Super's games. Though the "hows" of these effects may not be defined. For example a "Fire" effect could be magic (fireball), tech (flame thrower), advanced science (heat ray) or anything really. The aim is to describe what is happening and then GM/Players decide how it happened.
Also included are a list of Disabilities, things that can affect your character. This is very similar to other games, in particular GURPS. These grant a certain level of Character Points back to you. Finally we also get Action Points, which work like Drama Points in Unisystem.
The next part of the book are the rules of play, which is heavy on the combat and things like chases and mental battles. The rules are simple really, using the same basic rolls, just applied in different situations.
There is a section on adventures and a quick overview of character creation.
The Appendices cover different play modes. So for example Sorcery which adds "D&D/d20 Arcana" like spells to your game. Another is an expanded wealth system.
The art is sparse, and most of it is other Open art, but it is appropriate to the sections so for me it works.
Overall a solid game system to play or to mine for ideas.
[4 of 5 Stars!]