I'm awaiting delivery of an LG 55EG9100, but this question probably applies to most of the LG OLEDs. There are two audio outs on this, S/PDIF over toslink and HDMI/ARC. For the output, you can choose either "auto" or PCM. For "auto," the format will depend on the source and can be PCM, Dolby digital, or DTS/PCM. The way the menu is formatted, it suggests all of these are available on both ARC and TOSlink.

My question is whether anyone knows whether the TV mixes down the various channels to 2-channel PCM or whether it is sending multi-channel PCM? If it is 2-channel, which I suspect must be the answer for Toslink, does anyone know whether it is being intelligent in the mixing vs., say, just taking a pair of L/R and dropping everything else?

I want to put a cheap DAC on the toslink to add headphones or feed into an old analog Adcom pre-amp. The answer to the question determines whether that DAC needs to be a 5.1 channel to L/R or it can be a 2-channel DAC. For a 5.1 DAC, I'd probably use the Orei DA34 and for a 2-channel I'd probably use the Schiit Modi 2 Uber. I should say that I'm not trying to get great A/V sound. I just want basic function so that I can focus my resources on upgrading the existing 2-channel audio system for music in the future. (Please don't let that last comment distract us into A/V vs. hi-fi/audio :-) )
I would ask the question over at AVSforums.com they have a lot of members with experience and a thread on that TV.

Thanks for redirect me to AVS forums. It took a couple days to be registered, so I just posted a question there.
At 2:30 today my 65eg9600 gets delivered....I paid $4750 @ Best Buy...the prices are coming down
Paul, how did you get that price? That tv is still $5999.98 on the Best Buy website, never saw it on sale below that.
Must be the day....my 55EG9100 arrived today at 2PM.
lloydc....my wife and I saw it at the Best Buy store in Carson City NV for $5999 and it was on sale for $4999 about two days after Christmas. My wife did the on line shopping and found it for $4500 (plus shipping) from some brand X on line store. Best Buy has this price matching thing...we agreed to the $4750.

BTW, the price went back to $5999.

The picture is superb, far and away the best TV image I have ever seen

Paul, thanks.  You inspired me to order one, which is on the way.  BestBuy wouldn't match the online price I found because it wasn't one of 5 or so major retailers they recognize, so I ordered one from the company in NJ and hope for the best.

Subsequently learned the 65EG9600 won't display an HDR signal when they eventually release a 4K Blueray or Netflix streaming, because the HDMI inputs are not the latest HDCP-compliant 2.2a, and the HDMI port is said to be not upgradeable. The flat 65EF9500 is the one to get, but it is not discounted nearly so much at this point.  There may not be much significant HDR content for years, so I'm not going to worry about it.

A 55" was the right size for us, but it only comes as a curved display, not flat. I almost didn't order it for this reason, but now that it is here, I like the curved screen. I find that moving a bit one way or the other in front of the curved screen can reduce reflections a huge amount. At least, I think it is because of the curve, but others have complained the other direction. In any case, the picture is superb and I'm glad we got one.

Back to the original question:  I ended up punting and just getting a cheap DAC that handles 5.1 formats. The original idea had been to hope for a 2 channel and then get a "good DAC" that would also be useful for music. I'm glad to have gotten the cheap 5.1 DAC because it means I don't need to reconfigure the audio out on the TV when I want to use the headphones. So, the final configuration was to take the optical out through an active splitter, then to an Orei DA34 DAC, and from there to a little headphone amp. The other leg of the splitter goes to a sound bar. My objective in this setup was to direct the cash towards the TV and minimize expenditures on the TV audio so that I can use the saved money later for the audio equipment that I use for music.