Oktoberfest beer — or Oktoberfestbier — itself isn't strictly speaking a style of brew. Instead, Oktoberfestbier is now a protected geographical indication.
Only the beer that is served at the Munich Oktoberfest can be Oktoberfestbier, and for a beer to be served there it has to have been brewed within the city limits.
These restrictions mean that there are only six breweries that fit the criteria: Augustiner-Bräu, Hacker-Pschorr-Bräu, Löwenbräu, Paulaner, Spatenbräu, and Hofbräu-München.