Here and here (list is generated by Github based on the tags on the open projects) you can find long lists of open operations research projects on Github. By clicking on the "Open Issues" link on each page you can directly access the Github repository for that specific project. Note that @Kuifje's own repository VRPy is a great example.
There are lots of projects at COIN-OR, many of which I'm sure would welcome contributions. You would need to discuss with the individual project developers what the best way to help would be, but one thing that's nearly always welcome is help with documentation, including reference manuals, tutorials, example code calling library functions, etc.
VRPy is a python library for solving a range of vehicle routing problems.
It is open source and open to new contributors.
There are at least two ways to contribute:
solve one of the existing issues
have fun with it and propose new enhancements based on your personal experience with the library