If I had a couple of different OSS projects that I've contributed to I would probably put them in their own section titled "Open Source Projects" or something similar. If it's just one project I would probably include it in my personal projects section and be clear that it's OSS and provide details about my contributions.
Honestly, it seems to be pretty rare for newer Devs to be OSS contributors. Which is unfortunate because I think there's a lot to be learned from the process. I'm a decade in and there are times I still feel like contributing is too confrontational for me to enjoy it.
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