Oktoberfest in Munich does not require an entrance ticket. You don’t even need a ticket to enter the large and small beer tents when they are less crowded. However, a ticket is required to enter the tent when it’s closed due to the crowd or reservations, especially during the weekend and if you want to enjoy the Oktoberfest beer and food offerings.
Note: While Oktoberfest is free, Oide Wiesn and classic rides are not.