I think you're getting it wrong, drama fare like this is usually not going to have a major rush. Nolan's films usually leg it out, almost all have 3x or more with the exception of the Dark Knight Rises. So if it's going to underperform, then it's gonna come down to being a disappointment quality wise or opening low in the first place. But with the latter scenario, I just think the brand name and inflated ticket prices is probably gonna be enough to carry it to a solid opening as long as it's good.
Stuff like a Man Called Otto and Air make me more confident that despite dry subject matters, Oppenheimer can do fairly well. And sure it's long but audiences won't care so much if the movie's good, really. Hasn't stopped Nolan's other films from being long and still succeeding, plus the exact runtime is not locked down yet, so we're not entirely sure how close to 3hr it actually is.