Please help! Dark brown strands in stool. Could it be the oreos I ate?

Probably not. Some foods cause your stool to change color but oreos should not cause the stool changes you describe. I wouldn't worry if this issue occurred 1 time, but if you notice stool changes for more than a few days, see your doctor.
No. Oreos wil be digested by the time you have a bm. Any dark stool needs to be tested to make sure that it is not blood - you should see your doctor for blood test as well as rectal heme test to make sure you are not bleeding internally.