The regulator is to examine whether the show broke rules in the broadcasting code on references made to commercial products.
An Ofcom spokesman said: "We are investigating The X Factor for potential breaches of our rules prohibiting the promotion of products in programmes."
The media regulator is to examine two episodes of the show. On 17 October, when Vickers - a former contestant - performed, host Dermot O'Leary told viewers: "If you want to download Diana's single, the details are at ITV.com/XFactor." He made the same remark on the night of Canadian star Buble's performance.
Under broadcasting rules, products and services must not be promoted in TV programmes. The rule does not apply to programme-related material, which means the practice is legitimate when it concerns X Factor contestants but not when it relates to the promotion of guest acts.
A spokeswoman for the show said: "We are aware of the Ofcom investigation. It relates to an on-air reference regarding downloads of a guest artist which were not available on the website. This reference was as a result of a script error."