NBC’s “The Tonight Show With Conan O’Brien” is expected to dominate in demos when its first-week Nielsen numbers are released today — but week two may be a different story.

O’Brien’s overnight numbers have declined with each of its six telecasts following his big bow as host of the longtime latenight leader June 1. At the same time, David Letterman’s “Late Show” on CBS has been picking up steam by piling up big-name guests after laying relatively low for a while.

End result is that on Tuesday — for the first time in their seven head-to-head matchups — Letterman appears to have drawn more viewers than O’Brien.

In Nielsen’s metered-market overnights that measure the number of households watching a particular program in 56 of the country’s biggest markets, “Late Show” (with primary guest Julia Roberts) averaged a 3.4 rating/8 share to a 2.9/7 for “Tonight,” which featured Eddie Murphy.

The NBC latenight show had averaged a 3.1/8 on Monday, a 3.5/8 Friday and a 3.8/9 on June 4. It opened with a big 7.1/17 on June 1.

NBC has long said that O’Brien’s performance on “The Tonight Show” should be judged over a span of years and not days and that it believes O’Brien will prove to be the timeslot leader in demographic ratings that are key to setting advertising rates.

Still, the Peacock can’t like the downward ratings trajectory for “Tonight” and O’Brien to lose in any ratings category in an hour the net has ruled for years.

Household numbers don’t paint a complete picture of who’s watching and have become less and less relevant over the years as the audience has splintered. They tend to favor programs that skew older, and it’s likely that O’Brien — perceived to be edgier and younger-skewing than the more mainstream Jay Leno — has been losing more older viewers than younger ones as his household numbers have dropped.

Letterman would occasionally beat former “Tonight Show” host Leno in the overnights, but “Tonight” consistently won weeks and months in both demos and total viewers.

However, if Letterman ends up consistently pulling ahead of O’Brien in the overnights, it increases the chances that the CBS show will win in total viewers while challenging for the demo lead.

It didn’t help “Tonight Show” that NBC on Tuesday aired hockey live in primetime — and not entertainment programming — for the first time during O’Brien’s time as host. While the Stanley Cup Finals game did fairly well in the ratings, it may have disrupted some viewing patterns, and NBC’s West Coast stations aired local programming from 8 to 11.

The night was also atypical due to ABC’s coverage of the NBA Finals, whose down-to-the-wire Game 3 between the Los Angeles Lakers and Orlando Magic didn’t end until around midnight in much of the country.

That basketball contest earned a 5.9 rating/17 share in adults 18-49 and 14.2 million viewers overall from 9:02 p.m. to 12:04 a.m. ET), up a tick over Sunday’s Game 2 score but down a tick vs. last year’s Game 3 between the Lakers and Boston Celtics (6.0/18 in 18-49, 14.51m).

Happily for ABC, the Magic’s victory in Game 3 ensures that at least five games will be necessary to decide the best-of-seven series. The Lakers lead two games to one, with Game 4 skedded for tonight.

Game 6 of hockey’s Stanley Cup Finals, meanwhile, earned a 2.4/7 in 18-49 and 5.81 million viewers overall from 8:15-10:38 p.m. ET — the best numbes for the 2009 series despite competing for sports fans with basketball.

NBC can expect a pretty good number on Friday for Game 7 of the series between the Detroit Red Wings and Pittsburgh Penguins.