The traditional lager style formerly served as the standard Oktoberfest beer was Märzen.
Märzen is a Bavarian lager whose name refers to March, when it was traditionally brewed. Historical brewing restrictions made summer brewing difficult or unsafe, so brewers produced a stronger, more heavily hopped beer in early spring. It could then be stored in cool cellars and served later in the year, including at Oktoberfest.
Traditional Märzen was fuller-bodied, maltier and darker than the beer most visitors associate with the modern festival. It commonly had a deep amber to brown colour and enough alcohol and hop bitterness to remain stable during extended storage. The style became closely linked with Oktoberfest because the beer brewed in spring was ready for the autumn celebration.
Today, the official Munich festival beer is generally a lighter, paler lager called Festbier or Wiesnbier rather than the older amber Märzen style. That change creates a common mix-up: “Oktoberfest beer” can describe the festival beer tradition broadly, while Märzen identifies the historic style that preceded the modern pale version.