OEX is a file extension used by Opera web browser since version 11 released in 2010. OEX files store data (in the form of widgets) that add new functionalities to the browser. OEX files can be downloaded form Opera servers.

OEX extension files allow users to customize their Opera browser by adding new features and changing user interface according to their needs. Introduction of OEX format offered developers new opportunities for intergating their solutions with until-then closed architecture of browser used by circa 2% of global internet users.