A functional organisation structure can lead to a 'xxxx' effect whereby employees do not communi-cate...

Explanation

It's a 'silo effect'. Essentially this is about 'walls' being unintentionally created between functions of the organisation. Silos in the original sense are used for storing grain (tall thin buildings, common in the American mid-West); more recently for storing military missiles.

In the organisation this lack of ease in communicating can dent the performance of the organisation.

The other answers offered are irrelevant with Oreo being a type of 'much loved' American biscuit.

So anyway, 'silo'.