Make everything as simple as possible, but not simpler. -- Albert Einstein

OpenShift supports a bunch of programming languages. Here we take javascript/node.js for example.

Settings an env variable is as easy as one line in your terminal.

rhc env set NODE_ENV=production -a YOUR_APP_NAME

Then you can read the variable through process.env.NODE_ENV in your javascript file.

For more details, please refer to https://developers.openshift.com/en/managing-environment-variables.html