Additional resources for the assessment and enhancement of accessibility in Oracle Application Express apps include the following documents.
Oracle Accessibility Program - General information about accessibility at Oracle.
VPATs for Oracle Database related products - The most recently published VPATs (Oracle Voluntary Product Accessibility Template) for Oracle products. Oracle Application Express has published multiple VPATs.
Plug-ins for Application Express - Plug-ins developed by the Application Express team for use in your apps, such as the Simple Checkbox plug-in for an accessible single-option check box.
Understanding WCAG 2.0 - A guide to understanding and implementing web apps to the WCAG 2.0 set of guidelines.