Ive got some old Android phones laying around and I was wondering, if there is an open-source way of turning a old phone into a IP-Camera, that I can access from everywhere.
I couldnt find any relevant softwate besides Haven, which is not actively developed as of now. (and no commits since 2021)
And also, you cannot do a live-view afaik, only when the phone detects movement.
I could only find this closed-source software: https://play.google.com/store/apps/details?id=com.pas.webcam (IP Webcam)
Oh, thanks for Haven! Never heard about it before! 
I think that this Github issue is quite relevant: If someone was to restart Haven in 2025... · Issue #465 · guardianproject/haven · GitHub
There are plenty of options here I do think, most of them would plug themselves to OBS. For example this one: Turn Your Android Phone into a Virtual Webcam for OBS on Linux (Without DroidCam) - DEV Community
It of course depends on your setup, the phone, how you want to handle the battery, the kind of software you want to run on it (blueiris, frigate?) etc.
Otherwise, if you want something more powerful (referring to the “that I can access from everywhere” part here), I could recommend all the camera integrations from HomeAssistant: Integrations - Home Assistant
There are plenty of such questions on their forum/reddit too.
There is a popular and cheap IP camera, the TP-Link Tapo C200 that quite some people do buy and then restrict any access to the Internet thanks to their firewall rules.
It’s actually even a well-known challenge to buy the most shady camera and turn it into the most inoffensive self-hosted owned IP camera. 
Get a DynDNS like dynv6.com and Spydroid, connect the DynDNS to your device and you are ready to stream.
You could also use Endoscope for sharing your stream between Android devices. Should also work with DynDNS.
Both apps require at least Android 4.0.
Other app recommendations
Requires Android 7 or higher.
Requires Android 4.3 or higher.