From publisher blurb:
Take the guesswork out of creating new classes for B/X-style RPGs
The BX Options: Class Builder is the only race-as-class creation system 100% compatible with the classic fantasy classes presented in the core rules. Infinitely scalable for any setting or genre, the BX Options: Class Builder lets you create new (or modify existing) classes guaranteed to match the level progression rates of the canon.
Contained herein is everything you need to create perfectly balanced classes for B/X-style RPGs, including:
Class building system
30 variants of the existing classes
13 new sub-classes
Optional d6 skill system
A d6 conversion of the classic Thief skills
Optional spell failure guidelines
Animal totems and abilities
New armour and weapons
High-level options for advancing classes beyond 14th level