Although OChem has a lot of mechanisms, OAT only tests you on a limited number of them. Here is what they cover on the OChem section:
Organic Chemistry: Mechanism (Energetics, Structure, and Stability of Intermediates) - SN1, SN2, elimination, addition, free radical, and substitution mechanisms
Chemical and Physical Properties of Molecules and Organic Analysis - inter and intra molecular forces, separation, introductory infrared spectroscopy, 1HNMR spectroscopy, 13CNMR, chemical identification, stability, solubility, and polarity
Stereochemistry - conforma?tional analysis, optical activity, chirality, chiral centers, planes of symmetry, enantiomers, diastereomers, and meso compounds
Nomenclature - IUPAC rules, and functional groups in molecules
Individual Reactions of the Major Functional Groups and Combinations of Reactions to Synthesize Compounds
Acid-Base Chemistry - resonance effects, inductive results, and prediction of products and equilibria
Aromatics and Bonding - concept of aromaticity, resonance, atomic orbitals, molecular orbitals, hybridization, bond angles, and bond lengths.
If you want the complete listing of ALL topics that are covered in OAT you can find it here:
https://www.ada.org/oat/oat2003.doc