Broadcasting regulator Ofcom is investigating two instances when O'Leary told viewers they could go to ITV.com/XFactor to find details of how to download songs from Vickers and Buble.
Vickers, who got to the semi finals of 'The X Factor' in 2008, performed on the show on 17 October and the Canadian crooner Buble was a guest on the show on 24 October.
Under broadcasting guidelines 'The X Factor' can direct viewers to download songs from contestants in the show, as that is classified as "programme-related material" but it is not allowed to promote the work of guest performers.
A spokesman for Ofcom said: "We are investigating 'The X Factor' for potential breaches of our rules prohibiting the promotion of products in programmes."
A spokeswoman for 'The X Factor' producer, Talkback Thames, said it was aware of the Ofcom investigation. The spokeswoman said: "It relates to an on-air reference on 'The X Factor' regarding downloads of a guest artist on the show which was not available on the website. This reference was as a result of a script error."
Ofcom has received hundreds of complaints about 'The X Factor' this year, 606 in the week to Monday (6 December), but most have been to complain about decisions within the show, which are not governed by the Broadcasting Code.