Oracle University offers a number of courses on Application Express:
Numerous APEX Consulting Companies also provide training online, on-premise, or at their facilities.
Oracle Learning Library provides many Oracle by Examples (OBEs), Demos, Articles, etc for Application Express.
The Oracle Application Express curriculum is designed to introduce students to the design, development and deployment of beautiful, responsive, database-driven applications using the low-code framework, Oracle Application Express. The course content consists of presentations, hands-on labs, and required source files. This is available for download so that you may use these materials in your training courses or curriculum.
This same course content is also offered in a hosted interactive form by Oracle Academy, complete with quizzes. Oracle Academy also provides both educator and student curriculum. For more information, please visit Oracle Academy.