A few reasons behind the crazy drop for Oppenheimer I expect:

-IMAX driving 26% of the box office gross and still not able to meet a lot of the demand. Even if weekdays help, it will be more beneficial to support Barbie's supply constraints because not everyone has 3 hours mid week to watch it.
-Will be the must see movie next weekend as many chose Barbie first. Especially since it skewed 62% male expect Oppenheimer to get more female audiences in future weekends.
-3 hour run time so more will take their time to see it. Nolan's longest movie ever vs Interstellar (169 min), TDKR (164 min), TDK (152 min), Tenet (150 min), and Inception (148 min) vs Oppenheimer's 180 min.
-Zeitgeist movie so will pull in moviegoers that may have waited to otherwise see it. A watercooler movie that people need to see to be in the know.
-Well reviewed so could lead to repeats.
-OW skewed heavily to younger audiences 18-24 (33%). 18-34 (66%). Older audiences don't always go OW but there's a huge audience yet to watch it that sees Nolan movies.
-Light competition from holdovers and new openers with movies potentially under <$16M other than Barbie.

I think these are very compelling reasons to expect a great hold, but -15% is insane. You're basically predicting that Oppenheimer is the next (and bigger) Titanic.

Early reviews are starting to come in for Haunted Mansion and some mentioning weak visual effects.

It cost $158M. Did that mostly go to "star" budgets?

I looked at HM presales and seeing around 3.5-4m previews. Could hit low to mid 20s. Presales are not that backloaded. Last week both Barbie and Oppenheimer were really backloaded despite way higher presales.

Why on earth would it miss 30m when Jungle Cruise, the closest comparison, did 35m off 2.7m previews?

Even 3.5 would be great for previews

It's way too late but I still can't believe they never made a Wedding Crashers 2.

Owen, Vince - Careers have lagged so good opportunity.
Christopher Walken - Will get a boost post Dune 2.
Rachel McAdams - One of her post Mean Girls roles. Still has stayed fairly relevant.
Isla Fisher, Jane Seymore - Not big stars but familiar enough.
Bradley Cooper - Huge star who at the time was pre-Hangover.
Will Ferrell - Has stayed in the public eye.

Now adjusts to $298,649,171.