Opa is an open-source programming language for developing scalable web applications. It can be used for both client-side and server-side scripting, where complete programs are written in Opa and subsequently compiled to Nodejs on the server and JavaScript on the client, with the compiler automating all communication between the two. Opa implements strong, static typing, which can be helpful in protecting against security issues such as SQL injections and cross-site scripting attacks.