The cable network says the Tuesday edition of his new talk show, "Conan," delivered 2.8 million viewers. Of that total audience, 2.1 million viewers were in the 18-to-49 age group that many advertisers prefer.
O'Brien returned to late-night TV this week after nine months on the sidelines following his bitter exit from NBC last January.
His much-awaited debut Monday drew nearly 4.2 million viewers, beating every other talk show of the night.
"Conan" airs weeknights at 11 p.m. Eastern time.