The only reason that Dark Oak doesn't keep the link open is because she is about to kill someone. A Kandorian. And while she knows that Clark understands her job... she doesn't want him to experience it through the bond.

The job itself is quick and painless. She goes in shadow form. Use Kryptonite to weaken them. And snaps their neck, ensuring that they are good and dead before leaving.

Then reports back to Watchtower.