As I mentioned, they obvious did a digital intermediary for visual effects shots. For Oppenheimer I don't think they was much CGI, but at the bare minimum I sure they did some digital compositing, so they must have matched their photochemical color grading with digital color grading for some scenes. Also, they had a digital colorist to produce the grade for the DCP anyway, to match the look of the film master.
But it seems to me that reputable sources have clearly stated that they did a fully analog film path all the way to release prints.
There's this interview with the DP: https://fotokem.com/oppenheimer-hoytema-fotokem/
> And once production began, FotoKem printed literally everything Nolan shot. Then after the edit was complete, FotoKem brought in Negative Cutter Simone Appleby from the Centre National du Cinéma et de l’image Animée in France to cut each section and glue it together, which starts with a list that comes out of the Avid that provides every single one of the over 3,000 shots. That list, coupled with a work print (a film version of the list of shots), was then rolled against the negative, using also a QuickTime reference with key numbers—all of which allowed Appleby to cut the physical negative together into the final assembly, from which final prints were made. And from there, Van Hoytema and Color Timer Kristen Zimmerman painstakingly perfected the color of each and every shot by hand.
> Theodosiou’s focus was matching the color of the digital master to that of the analog film master because Nolan and FotoKem knew that not everybody in the world would be able to view Oppenheimer on analog film. “We were able to manipulate the images and make them look exactly how Christopher intended for them to look on film,” says Theodosiou. “For the final tweaks, we had to split screen the image between a digital and a film on the screen and match the film identically throughout, from reel one all the way to the end.”
