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

Final Nielsen numbers released Thursday showed O’Brien is connecting in a big way with the young adults that NBC was hoping to attract 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.

According to Nielsen estimates for the week of June 1-5, “Tonight” averaged a 2.3 rating in adults 18-49, a 156% advantage over CBS’ “Late Show With David Letterman” (0.9) — the most lopsided difference between original segs of the two shows 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 still would have given “Tonight” its largest victory in 10 years.

“Tonight” opened up a 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).

Although O’Brien fell behind Letterman in the overnights Tuesday, the new host was expected to dominate his premiere week in key demos as well as total viewers (Daily Variety, June 11). And with the finals bearing out that O’Brien’s overnights will translate very well in the demos, it would appear the NBC entry merely would need to be competitive in the overnights to notch an 18-49 victory.

On Wednesday, “Tonight” edged past “Late Show” in the overnights (3.1 household rating/8 share to 3.0/8).

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). Fallon has won or tied for the demo lead on 68 of 70 nights since bowing as the host in early March.

Fallon also prevailed for the hour in total viewers (2.4 million to 1.8 million for Ferguson).