You'll also need to learn CodeIgniter and Bootstrap. CI is the PHP framework that OSPOS was built on top of. Bootstrap is a popular frontend framework that v3+ uses.
You can learn CodeIgniter by reading through the documentation, or if you want a quick, general overview, you can read CodeIgniter: Learn By Coding by Scott Hughes. This book walks you through creating a basic sample app in CodeIgniter. After reading it, you'll have a good understanding of the framework, which will help you understand OSPOS's codebase much better..
Make sure you have set up your server correctly before you start developing. Read the README.md, specifically the Server Requirements section. We recommend DigitalOcean for your server. Please read our installation guide. If you sign up with our link, you'll get a $50 credit.
After you or someone else has made changes to OSPOS, consider creating a new pull request at the official OSPOS repository. That way your changes won't be overwritten if you update the software in the future. Plus, you're giving back to the OSPOS community.