Google has recently started detailing every Android Wear watch that will be getting the Oreo update and if you own one of the most recent watches that have made it to market over the last year then you may already have the new software installed. If your watch is a little bit older, then that doesn't mean you won't be getting Oreo, but it's also not a guarantee that you will. The list also seems to be consistently updating and will probably continue to do so until every watch is either confirmed as having it or not getting it.
In regards to which smartwatches are currently pursuing the Oreo software update, this includes both models from CASIO, the Fossil Q Control, the Huawei Watch 2, the LG Watch Style, the Misfit Vapor, the Mobvoi Ticwatch S & E, the Nixon Mission, the Polar M600, and the ZTE Quartz. There will be a number of watches that aren't getting the update to the software based on Oreo, and this is any watch that Google didn't update to Android Wear 2.0. If you have a watch that didn't officially get that software from Google then the update to Oreo will no be coming through to the watch you own.
That said, every other Android Wear smartwatch is listed as already having the software update, which includes watches like the Fossil Q Venture, the Mont Blanc Summit, the LG Watch Sport, the DIESEL Full Guard, the Fossil Q Explorist and many others. If you're particularly interested in seeing the full list of watches that have the Oreo software currently you can view the full list at the Android Wear Help page. As for what comes with the new Oreo software, numerous new options and features are included like the vibration pattern for incoming notifications, newly support languages and countries, a touch lock option, notification channels and more. It should also include the new dark theme that was discovered not too long ago if you prefer this over the standard color theme that is applied to the watch menus and system. If you haven't received Oreo yet and you have one of the watches that's listed as having it, it's possible that you may either need to try connecting the watch up to the internet again if it hasn't been connected in a while, or it may simply be continuing to roll out to more users in batches.