I have a Godox TTL 350o, if I bought a 350N can i use the 'o' model as a triggered external flash?
TT350s can trigger each other cross-brand, but you might not get TTL or HSS.
Unlike the TT685/V860II, the TT350/V350 are not built for cross-brand TTL. I discovered this when I tried to trigger a TT350-O off-camera from a TT685-C on my Canon 5DMkII. I have the Flashpoint branded versions from Adorama, and their support person emailed me when I mentioned the TT350-O was only firing at full power in TTL (remote power control and HSS worked fine), and said that the TT350 is not designed to do cross-brand TTL.
Different flavor combinations have been reported as working in TTL on dpreview (Sony, Nikon, iirc), so I would test to see.
Most likely yes (supposing that the 350N acting as a commander will be mounted on a Nikon camera). The brand-specific variants of each model (Nikon, Canon, Sony, Olympus and so on) only matter because of the different hotshoes. Their bigger models that do not feature a hotshoe (AD200, AD400, AD600) are all brand-agnostic and work regardless of what's the camera brand as long as the commander matches the camera.