O is for two games that are related. Open Core and Open Anime.
Both games are from Battlefield Press and both use quite a bit of the SRD, but they are not d20 or D&D.
Both are designed to allow the GM to create his/her own game with the tools provided. Both also use the same basic roll system; 3d6 + Attribute + Skill. (or even 1d20 + Attribute + skill). The system is modular, so you can take or leave what you like.
Open Core is based on the OGL but differs quite a bit from it. Like a number of game, 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.
Open Anime is a little different. It is very, very similar to Open Core, but with more of an anime overlook. What does that mean? Well, for starters we start with the various Anime sub-genres and how these change how you view and how you play your character. If Open Core is the Marvel Universe then Open Anime is the Marvel Manga Universe.
Open Anime is actually a game I have wanted to write. When BESM disappeared there was a vacuum for a good Anime RPG. With all the Open systems out there and the BESM-d20 system open, all the elements needed are out there. Open Anime combines all of these using Open Core as their guide. While I don't think it quite reaches the heights that BESM does/did, it is a rather good effort. Good enough that I don't have any desire to write my own Anime game anymore, I'll just tweak this one. There is a heavier emphasis on Magic and Magical Girls than say other types of Anime Genres. Which I think is a little disappointing.
What gets me the most about this one is there seems to be a lack of what makes an Anime RPG an Anime RPG. I think in the attempt to make this game more open and more flexible they lost a little of what makes Anime different than some other games.
Both games are fun and I'd like to see more from them.