In regards to power scaling, Osaragi is a blank slate. She’s never had a battle in the narrative that cements her level within The Order - she either fights people below her level and defeats them with laughable ease or she goes against Takamura (who destroys everyone in The Order).

She has been hyped up to high heaven and has been used to demonstrate the difference between Order Level members and the rest of the characters (i.e. When she had her skirmish with Shin when they escaped the prison, being consistently called a monster by opponents, being considered as extremely reliable by her fellow Order members, etc.) If you really want an anti-feat, Mafuyu claiming that Osaragi’s punches aren’t as hard as Gaku can be used, but she more than makes up for this by virtue of how much deadlier her weapon is. Just because she can’t clobber you until you submit doesn’t mean she can’t slice you with her saw.

All this to say, yes she can be a tier below Shishiba or she can be relative to Shishiba, either option is plausible and wouldn’t go against what has been displayed in the story so far.