Is the Oktoberfest cash-only or are credit cards accepted?
Generally, cash is king at the Oktoberfest. You may be allowed to pay for a whole reservation using a card at the end of the night, but that may only include debit cards, no credit cards. Apart from that, the acceptance of cards is still very low. You can't expect individual orders to be paid using cards. You may be able to pay at some stalls or rides using cards, but you can never count on it.
The only thing you can rely on is cash.
There are ATMs at the fair grounds as well as in all big tents. However, most of them aren't operated by banks and come with ridiculous charges. Therefore, it's better to get cash before getting there.
If you're from a non-Euro country, check your bank's fees on getting foreign currencies from an ATM abroad. It's usually best to get foreign currencies directly from an ATM instead of exchanging cash or getting it back home. Make sure to use a bank's ATM and chose to be charged in Euro instead of your currency. This way you can avoid external fees.