4:3 is the correct aspect ratio for IMAX but it sounds like everything else about the presentation at the theater was wrong so who knows if there were any other issues at your particular theater. The movie was released in a number of different formats so there's already a cropped version that will go onto the disc https://www.reddit.com/media?url=https%3A%2F%2Fpreview.redd....

Unfortunately they emptied out the proverbial bench to get enough staff worldwide to play Oppenheimer in film formats wherever possible and that led to some pretty spotty results.