Use TeKa A & B
I did the same thing to some hatch boards that I was refinishing. The interior side was oiled. I used a two part teak bleach, TeKa A&B. Very caustic but very good. You wet down the boards, spread out the A, wait a few minutes, then scrub lightly with a 3M pad. The teak will turn very dark, almost black. After the A, you spread the B on until the wood returns to the original blond color. Be generous and watch for bleed back (were spots turn dark again). After the B, rinse well with running water and let dry for at least 24 hours.This will raise the grain so some hard sanding will be necessary. Once sanded down your ready for varnishing. I recommend a Tyvek or painters suit and rubber gloves. Not good on the clothes or skin.Enjoy,Les Murrays/v Ceilidh86 C-36 #560