AppNexus CEO Brian O'Kelley is stepping down, the first organizational change since AT&T acquired the ad tech firm in June.

This comes as AT&T's advertising and analytics unit, Xandr, looks to integrate AppNexus with its team. O'Kelley will take on an advisory role with Ryan Christensen, senior VP of product overseeing all existing AppNexus products and new digital platform products, according to an internal memo from Xandr CEO Brian Lesser.

Layoffs are not expected as part of the integration, according to a person close to the situation.

In June, AT&T acquired AppNexus, which O'Kelley started more than a decade ago, as it works to reinvent TV advertising to become more targeted and efficient and challenge digital behemoths Google and Facebook.

AppNexus predominantly serves as a sell-side platform, meaning it provides publishers with software to automate ad sales in formats such as display, audio and over-the-top video. The company also works with marketers to help them buy ads, competing with companies like The Trade Desk and Media Math, though that portion of its business is regarded to be much smaller than the sell-side business with publishers.

With AppNexus, along with the content it acquired from Time Warner's networks like TBS and TNT, AT&T is looking to build out a programmatic exchange of sorts for the TV marketplace.

Contributing: George Slefo