Expired Oreos can have a variety of flavors, depending on how old they are. In general, they will be far less sweet and much more dry than when freshly made. You may also find that the texture is grainy, which is caused by moisture leaving the cookie over time or through contact with air. Additionally, expired Oreos may taste stale due to oxidation of fats in the cookie which occurs as it ages or through exposure to light and air.
That said, every person has their own unique experience when tasting expired Oreos as our individual palates differ greatly - some may not even notice a difference while others will be able to pick up on more subtle flavor changes from fresh ones. Ultimately though, if you’re concerned about eating them - it’s always best practice to err on the side of caution and throw out any expired food items!