"And indeed the Wikipedia article to which you link makes it clear (in the second paragraph) that the central bank funds its open market operations by printing more money."

The wiki states: "To pay for this, bank reserves in the form of new base money (for example newly printed cash) is transferred to the sellers bank"

Granted that is one example, but the US fed is already flush with currency and doesn't require printing any more to meet demand, they simply transfer 'elastic currency' in and out of the money supply.  The treasury has the sole authority and responsibility of printing currency.  In the US, the Federal Reserve can not print currency to increase base money.

http://www.investopedia.com/articles/economics/08/treasury-fed-reserve.asp