Both follow a group of high school best friends who are impacted by a tragedy. Through past (high school) and present (adult) timelines, a supernatural "time-slip" reunites them as they try to fix the past and prevent their biggest regrets.
Additionally, both are set in Nagano Prefecture: Orange in Matsumoto and Boku Dake ga 17-sai no Sekai de in and around Lake Suwa.
Plus, Yamazaki Kento appears in both (but just a guest role in 17-sai)!

These two TBS dramas are definitely cousins!
Both are slice-of-life dramas that address the modern malaise of trying to find your place in a world where you don't quite fit in, all while navigating the ups and downs of love and life.
In both dramas, the female lead is a quirky young woman who often over-thinks and over-analyzes situations, which sometimes puts her at odds with other characters/society. Unsatisfied with her career, she decides to make a major life change early in the drama. Both are “everyone has their own story” kind of dramas where even the most minor character is important.