Disconnecting the HDMI and using a different power outlet for the psone made no difference. I should be able to get my hands on a Gamecube RGB cable sometimes next week hopefully.
I connected a Wii and a PSPgo over the AV2 though and both synced immediately.
I also stumbled upon this thread earlier today: https://videogameperfection.com/forums/topic/nosync-japanese-psone/.
His observations are pretty similar to mine, are you definitely sure that the OSSC should work with a PSone? (the slim model)
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