BX Advanced Bestiary
Volume 1 of the BX Advanced Bestiary takes the monster entries A-D from the OSE Basic book and expands them with options, added details, use for monster parts, additional monsters, and playable classes based upon the same theme. A sample preview of the book can be found here. Each entry is illustrated.
It includes rules for monster and animal training, several new race-as-class options (advanced cave locust, changeling, cyclopskin, dragonborn, and genasi), as well tables showing the monsters organized by HD, terrain, and type.