What device are you using to capture audio? The most likely culprit is dual path kida stuff, where your software is giving the audio to OBS and also to a USB device, and then the USB device is also giving the audio to OBS.
I had an echo like this when I used a USB Mixer to capture my microphone for the Zoom, and to pay back the caller out loud.
A good place to look is in OBS' Preferences to make sure no audio devices are automatically assigned.
If one of those is contributing audio, it might not show as an audio device in the mixer zone, but Zoom might be channeling it's audio through one of those!
Good Luck. I hope it gets sorted.