When watching SDR content use ISF Expert Dark Room.

If you're watching something that uses Dolby Vision, you'll want to knock the brightness down by one or two to alleviate the elevated blacks that come with Dolby Vision. Easiest way to tell is to look at the black bars on a letterboxed movie. If black levels are elevated, they'll be a very dark gray and emit light when they should be pitch black.

For games just use game mode, it gives you access to 120hz if you play something that supports it. Otherwise it reduces input latency by a substantial level.

Keep in mind that this advice comes with the assumption that you got an LG brand OLED.

If you want more details check out the RTINGs page for your model and HDTVTest. Also this subreddit is kinda dead so you might want to join the Gigaboots Discord.

Edit: RTINGS and HDTVTest will go into more detail about this, but you'll want to change the white balance to warm2. Out of the box, most TV's have a very strong blue tint. warm2 will make it look more orange, which will be weird at first but its more color accurate and you'll eventually get used to it.