Is there a shuttle from Clinton to Oak Harbor? If not, what is the best way to get from the Clinton to Oak Harbor?

There is taxi service on the island but expect a hefty fare for a 45 plus mile trip. No Uber or Lyft. Bus is your best bet. Free, safe, on schedule, except Sundays, as noted above. You could try Rideshare. Those vans are on the ferry every single morning.

There is a bus that runs from Clinton to Oak Harbor it's free but doesn't run Sunday. The shuttle goes from Oak Harbor to Seatac and is around 80 dollars a trip.

I know there is a shuttle from sea tac to Whidbey island. I don't know about oak harbor. I bet you could Google the Whidbey shuttle and it would tell you