Branson, who met Competition Commissioner Joaquin Almunia this week to discuss the issue, said Virgin would attempt to show why the alliance should be stopped.
The Commission opened an investigation into the planned alliance last April.
The airlines want to manage jointly schedules, capacity and pricing as well as share revenues on routes between North America and Europe.
American Airlines, BA, Iberia, Finnair
The DOT has given interested parties 45 days to object. Answers to objections will take a further 15 days.
BA shares were up 1.4 percent and Iberia was 0.4 percent higher by 2:33 p.m. British time, outperforming a 0.33 percent lower Stoxx Europe transport and leisure index <.SXTP>.
The Commission is also probing proposed alliances between members of Star Alliance, and members of SkyTeam, both rivals of Oneworld.
(Editing by Dale Hudson)