I've been thinking about your conundrum since seeing it over in kitchens... first, my sympathies.

Have you considered cutting a groove where the oak is and gluing in a complete stick of maple? If you left the 'new' maple proud, then you could go over it again with a bearing guided router bit and have a nice finish. And with a good water-resistant (like titebond 2/3) glue, you'd have better water protection than the existing gaps.

Good luck with this one.

That's not a bad idea.. use a rabbeting bit and router and cut out a groove where the oak is.. from above the top and bottom?

glue in a strip and re-route flush.. do you agree on the method to remove the oak I described above?

I think I agree on the method, but am a little confused by the phrase "from above the top and bottom".

Can you still remove the countertop? If so, your task will be much easier, as it would take a bit of fiddling to get a rabbetting cutter that would cut down far enough while having enough of the shaft engaged in the collet.

I know you know this, but for the sake of newer readers, take small bites and repeated passes with the router, especially with the big bit described here.

I am still considering this idea, any idea how to remove the rounded cut at the ends left by the rabbeting bit? Would a chisel be good enough?

Do you think it will hold up when it is only glued on one edge? I need to put some masking tape so any glue squeeze out cannot soak in the unfinished wood and affect the finish right?

BTW I think a 3/4" router bit will be better for the job, I don't need the cut to be very deep.

I read about titebond 3 is supposed to be more water resistant.

The rounded cut deep inside the groove should be easy with a chisel. (It's invisible, so a little bit of butchery would be okay.)

If it was me, I'd want the groove at least 1/2" deep, which would give gluing surface on the back as well as the top and bottom. I think that would be more than adequate, especially given that the inserted piece is under no stress.

Masking tape might help with glue squeeze-out. This is just my personal take on it, but I'd sand lightly afterward and not worry about glue residue marring the finish. (Maple isn't very porous, so the glue should stay on the surface.) If anyone else out there has differing opinions, please comment, as I'm not a finishing expert by any stretch.