There are some general subjects that will show up in biology courses and competitions that can't really be clumped into specific subjects, such as statistics and chemistry. These, along with other general resources are linked below.
Our handout on Biostatistics addresses most of the statistics you need to know for USABO.
Relevant topics in chemistry are explained in this link from Sparknotes. Make sure to read about stoichiometric calculations, molarity, reaction kinetics, and acid/base reactions (especially with buffers).
The Guide to IBO discusses many theoretical and practical aspects of the IBO, many of which pertain to USABO.
This is a partial outline of Campbell's made by Varun.
This is a partial outline of Campbell's made by Shulin Ye, a previous member of Team USA.