OK, there's a lot to unravel here since it sounds like you're in seriously uncharted waters.
First, OSSC does not support CVBS Composite, AKA "the yellow wire" or Y/C component AKA "S-Video." For NTSC users like me you might have to struggle a bit to get a supported signal into the unit. Luckily most analog retro consoles either already output the proper video signals with the correct cable or they will after a bit of light modding. The results are totally worth it!
On its own OSSC only supports RGBS, RGsB, RGBHV, and YPbPr. Typically, that translates to RGB Sync over SCART, Sync on Green with PS2/3 SCART/component cables, VGA over 15-pin D-sub, and HD component over RCA Cinch. If you want to add CVBS and S-Video inputs you'll want something like a RetroTink 2X with an active HDMI to VGA adapter, but then you might be happy just running the HDMI output from that without bothering with the OSSC.
As for audio, the original DVI version of the OSSC only had an analog audio pass-through which would break out the SCART audio through a headphone jack. You were expected to connect any other audio source directly to your stereo or TV. Of course, many displays will not have analog audio for digital video inputs but it was common when DVI was still found on HDTVs.
Thankfully, the current OSSC with HDMI instead of DVI now supports analog audio and digitizes it together with the HDMI video signal. They even added (or repurposed?) a 3.5mm TRRS "headphone" jack for stereo input. Because it's open source, my DVI version was even upgraded with the audio board from Borti. This upgrade board digitizes analog audio from the SCART. The TRRS jack on mine is still an output so I use this SCART plug thing to connect discrete analog audio with typical RCA Cinch connectors:
The S-Video and composite ports won’t do anything since this just passes through to the SCART connectors where the OSSC already doesn’t do anything with composite or S-Video. The OSSC will get audio from those SCART connections or the TRRS input regardless of what video input you have selected. As an audio input this will work for you too but, obviously, we can’t connect a SCART device at the same time so you'll probably be better off with a cable similar to this:
You'll want some RCA Cinch gender changers on hand, of course.
I'll list a bunch of systems and how you can connect them with minimal modding:
Atari 2600, 5200, 7800, and XEGS/8Bit: RGB mod with UAV and a custom cable
NES: Upgrade with NESRGB and use SNES HD Retrovision (HDRV) cables
Master System: Use Genesis 2 HDRV cables with the Master System adapter or a SCART cable
Genesis 1: Use Gen2 HDRV cables with the Gen1 adapter or a good SCART cable
TurboGrafx-16: Use dbGrafxBooster/Engine Block with Gen2 HDRV cables
Turbo-CD or Turbo Duo: Use Voultar's RGB amp/replacement DIN and a SCART cable.
Neo Geo AES: Use Gen2 HDRV w/adapter
SNES SNS-001: Use SNES HDRV cables or good SCART cables
3DO: Must custom install a mod board for the cable of your choice
Neo Geo CD: Use Gen2 HDRV w/adapter
Saturn: Use SCART or Gen2 HDRV w/adapter
PlayStation: Use SCART or Gen2 HDRV w/adapter
N64 (NS1 serial): Install Voultar's amp and use SNES HDRV/SCART
SNES SNS-101: install Voultar's SNESRGB bypass board and use SNES HDRV/SCART
N64 (NS2 serial): Install Borti's or eTim's RGB board and use SNES HDRV/SCART
Dreamcast: Use SCART or VGA box with cables
PlayStation 2: Use original or HDRV component cables for RGsB/YPbPr (menu option)
XBOX: Use Advanced AV Pack with generic HD component cables
GameCube (NTSC DOL-001): Use Nintendo component cables or recent clones
GameCube (NTSC DOL-101): Use GCDual with SNES HDRV... or just go HDMI directly
XBOX 360 (analog-only versions): Use component cables... or upgrade to a later model with DVI directly
PlayStation 3: Use original or HDRV component cables for RGsB/YPbPr (menu option)... or just go HDMI directly
Nintendo Wii: Use Wii component cables
Nintendo Wii U: Use Wii component cables... or just go HDMI directly
So, I have to ask: what were your intentions when you bought it? If you just want a decent quality, low-latency, HDMI converter that doesn't destroy 240p effects from retro consoles then you might be better off with a RetroTink 2X. This is especially true if you just want to stream but don't like the price or other aspects of the Framemeister XRGB Mini.
Because OSSC is all about preserving the original quality without adding any latency. It can't buffer frames so it can not make the digitized video conform to proper HDMI specs. As a result, the signal and is incompatible with most HDMI capture hardware and streaming devices. Heck, virtually no console outputs exactly 60hz for standard HD timings so many TVs (like mine) only work in Line 2X mode... making it even more like the RetroTink 2X.
https://imgur.com/a/d2NouI5...