Skyblivion is a massive project that intends to port Oblivion into Skyrim's advanced engine. The modders are planning to port all environments and quests over, though in its final form you'll be able to pick and choose the parts you want.
"This includes all enhancements developed by the team such as our Landscaping Overhaul, Interior Overhaul, City Overhaul, Weapon/Armor Overhaul and much more," reads a post on the Skyblivion site.
The project has been in development for years. Back in February we got a look at the Imperial City at the heart of Oblivion's map. For an earlier work-in-progress view, check out one of Skyblivion's teasers from back in 2016.
The final mod will be free, and is designed to be compatible with the Skyrim Special Edition. You'll need to own Skyrim, Oblivion and their DLC packs to install the final mod. There's no release window at the moment. It's a volunteer-driven project, so progress can be irregular, but it's come a long way since Skyblivion started way back in in 2012.