Leonard was the 8th overall pick of the 2023 NHL draft. 8th overall picks are generally not superstars. Ignoring the last few years as too soon:
2019: Philip Broberg
2018: Adam Boqvist
2017: Casey Mittelstadt
2016: Alex Nylander
2015: Zach Werenski
Against that standard, I'd put Oshie as arguably first (with Werenski making for the arguably). Of course, you go back one year further, and you get William Nylander (but I digress).
Having said that, you are underrating Oshie. Since he entered the league in 08-09, Oshie is 34th in goals and 47th in points. If you consider that there are 96 first line forwards in the NHL, that squarely puts Oshie consistently on the top half of first line productivity.
For what it's worth, Leonard's year has me hoping he can exceed Oshie's numbers in the NHL but reaching Oshie's level would still be an incredibly good result.