In the nine years since he founded ad-tech firm AppNexus, CEO Brian O’Kelley has established a reputation for blustery public pronouncements against the power wielded by digital ad-selling giants like Google. But until recently AppNexus didn’t have the weaponry in the form of digital ad selling software systems to be competitive with Google for business.

AppNexus, which operates software systems for real-time auctions and placements of Internet advertising, hopes that is starting to change. Recently, it rolled out a new suite of software to help websites and apps sell their ad space. That allowed it to broaden its client base, which in the past was limited mostly to advertisers looking to buy ad space. As a result, Mr. O’Kelley says AppNexus has won publishers away from Google’s DoubleClick for the first time in a decade. (It won’t identify those clients although AppNexus says it picked up Le Figaro, a French newspaper, which wasn’t on DoubleClick.)