Conan O’Brien continued to look good in his second week as host of NBC’s “Tonight Show,” racking up impressive demo victories.

His advantage over chief rival David Letterman on CBS in total viewers, meanwhile, is looking tenuous and it appears that both may end up claiming victory in some weeks.

In the demos that drive the biz, though, O’Brien has cruised by surprisingly large margins. Including the current week, the NBC host has won his first 13 nights in 18-49, and most by at least half a ratings point.

Looking at the Nielsen numbers for June 8-12, “The Tonight Show With Conan O’Brien” averaged a 1.5 rating/6 share in adults 18-49 (translating to 2.013 million viewers) vs. a 0.9/3 for the Eye’s “Late Show With David Letterman” (1.126 million).

NBC notes that excluding O’Brien’s big premiere week, this 79% margin of victory in 18-49 viewers for “Tonight” over a firstrun week of “Late Show” is the largest in the five-plus years since the frame of the “Friends” finale in May 2004.

O’Brien may be taking away some of Letterman’s younger auds, as the NBC host delivered a huge 177% advantage in adults 18-34 viewers (1.04 million to 376,000). The median age for O’Brien’s aud during last week was 45.7, compared with 56.7 for Letterman.

The CBS host was boosted last week by guests including Howard Stern and Julia Roberts as well as the controversy surrounding Gov. Sarah Palin. In total viewers, “Late Show” pulled within 100,000 viewers of “Tonight” (3.7 million vs. 3.8 million) — its most competitive position opposite a firstrun week of “Tonight” in 3½ years.

And with a big total-viewers victory Monday — thanks to Letterman’s second apology to Palin — “Late Show” is expected to prevail for the week by this measure.

Numbers for ABC’s “Jimmy Kimmel Live” were a bit murky due to basketball-related delays on Tuesday and Thursday, but the net said Kimmel had its most-watched summer week in three years (1.68 million). And on Thursday, Kimmel delivered more 18-49 viewers than an original hour of Letterman for the first time ever (1.15 million to 1.11 million).