Conan O’Brien’s opening-week ratings victory on NBC’s “The Tonight Show” proved to be even bigger than expected in key demos.

Final Nielsen numbers for the June 1-5 sesh revealed that O’Brien is connecting in a big way with his young-adult target aud in the 11:30 p.m. hour. In fact, among adults 18-49, the new host delivered the show’s highest-rated frame in more than four years.

“Tonight” averaged a 2.3 rating in adults 18-49, a 156% advantage over CBS’ “Late Show With David Letterman” (0.9) — its most lopsided decision in a week with all originals in 17 years. And NBC notes that if you toss out O’Brien’s big bow on June 1, his four-night average for the week would still have given “Tonight” its largest victory in 10 years.

“Tonight” opened up an even bigger 267% advantage over “Late Show” last week in adults 18-34 (2.2 vs. 0.6) while also winning by 74% in total viewers (6.1 million to 3.5 million).

O’Brien’s success helped boost Jimmy Fallon to a record rating in 18-49 as host of NBC’s “Late Night” (1.0), well ahead of CBS’ “Late, Late Show With Craig Ferguson” (0.6).

— Rick Kissell