After publication in the OG, there is a 30-day period in which the public may object to the registration of the mark by filing an opposition. An opposition is similar to a court proceeding, but is held before the Trademark Trial and Appeal Board, a USPTO administrative tribunal. A third party who is considering filing an opposition may first file a request for an extension of time to file the opposition, which could delay further action on your application.
The next step after publication depends on your basis for filing the application: