OpenACH is developed on top of Yii, a high performance, secure PHP framework geared for developing Web 2.0 applications. For those not familiar with this framework, nearly all web application code is found under the /protected/ folder. Yii organizes the webapp code in a MVC (Model-View-Controller) architecture, and as such the code is neatly organized into sub-folders /models, /views, and /controllers, respectively. Configuration files are under the /config folder, and core components on which the application is built are located in the /components folder.

Portions of OpenACH's core processing are designed to be reusable and are organized into two sub-projects located in /protected/vendors/. In that folder, the OpenACH sub-project contains the OpenACH SDK, NACHA record and file building framework, and various bank plugins. The OpenData sub-project contains the core record parsing and formatting libraries, as well as data aggregation and helper libraries, all based around publicly available databases.