Most common is watching your own stream from the machine you're streaming from (including the Twitch Dashboard, which has a 'preview' that plays audio). Results in an audio loop around 15 seconds long.
Chorus/echo audio is second-most common, and mostly happens when someone sets their mic to their webcam's audio device, leading to mic audio coming in both over the cam and mic channels. Fixed by setting the webcam's audio device to Disabled.
After that it gets into esoteric things like virtual cable setups, 'listen to this device' errors, and sound card weirdness.
No one here will charge for help by the way; though if you want to donate to the OBS project in general, there's a link up at the top of the page. Really though, post a logfile from the Help menu, and if possible link to a VOD showing the problem in action. Logfile first.