I thought it was wild that Oshi no Ko started with a 1.5 hour movie. I wonder if this is a new trend. Either way, to me this speaks of the level of quality they are aiming for in adapting the series, so it makes me more excited for it.
The thing with Oshi if they didn't make the first ep 90 minutes it made it would've made the show lose nearly half it's eps just to get to the "scene". It helped make the scenes flow narratively instead of chopping it up into 3-4 eps. Personally if shows wanted to do this to pack the exposition/party building into one of their eps I'm all for it.
I was thinking about that the other day. It must have been a hard sell to get that 90 minute episode in. It makes sense, to anchor the characters to their mother and provide a driving motivation to them, without giving the viewers a bait and switch.
The justification here is probably just that "wow, look how popular Oshi no Ko was, let's do what they did".
I had originally thought the series would cover the first 3 volumes, with the Demon Infiltration arc to serve as its climax, but if they are doing a movie that I assume will cover all of volume 1, I wonder how far they intend to go in this.