Teams first had to notice that there were three varieties of Oreo in the package. There were Double Stuf Oreos, regular Oreos, and Oreo halves with no stuf. These corresponded to dashes, dots, and character breaks respectively in Morse code:
- . - . | . - . | - . - - | . .
. | - | . - | . - . . | . . . | . -
- . | . - . | . . | - . | - - . | . . .
or, more specifically, the Lower Crystal Springs Reservoir. There's only so many letters that one can encode in cookies.