In Grover's algorithm, the oracle function marks the target elements in the search space. The oracle function should flip the phase of the target elements. Here's a simple example of what the oracle function might look like in Quipper:

```haskell
oracle :: Qubit -> Circ ()
oracle q = do
    -- Apply X gate to flip the phase of the target element
    controlled_not q (head qubits)
```

In this example, the `controlled_not` gate is used to flip the phase of the target element when the qubit `q` is in the |1⟩ state. You would need to customize this logic based on your specific target elements and search space. Let me know if you need further clarification or assistance with the oracle function!