The figure does not include viewers in other countries, or the 1.1 million who watched a repeat immediately after the initial broadcast of the 90-minute special that was also streamed live on Oprah.com.
Entertainment Weekly called the ratings "extremely strong," but showbusiness trade journal Variety thought the numbers were "below what might have been expected considering the gravitas of the interview."
TV ratings authority Nielsen also put viewership at 3.2 million, adding that the "Oprah's Next Chapter" special "ranked among the top 20 most-watched primetime programs for the night among all broadcast and cable networks."
Thursday's much-anticipated interview ran against American Idol on the mainstream Fox network, which drew 16.2 million viewers during its two-hour broadcast, Variety reported.
Part two of the Armstrong interview, pre-recorded on Monday in his hometown of Austin, Texas, goes out on the OWN network and online from 9pm Eastern time (0200 GMT Saturday).