I do not see a gap and why an OTR MW when you have an outside wall for venting.Why no cabinet on the other side of the window, why 2 mismatched floors , there are a few issues I see but no gap.

We decided not to have a cabinet on the other side of the window because we had to remodel the kitchen from a previous version which had no sufficient cabinets and space. This was done by extending the kitchen space towards the dining area and we thought that a cabinet again on this side will make the dining look smaller as we already lost some dining space

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Why wasn't the cabinet hung flush with the adjacent cabinet? Just move it.

I am planning to move the cabinet on the other side of the oven also further towards the wall to get the gap the same on both sides. But how would I fill them?

@houssaon We could not open the oven door properly because it was touching the cabinet door, so we moved it away towards the window.

The gap is around 1.5 to 2 inches.

I assume these are IKEA cabinets.

Loosen the left-hand cabinet from the wall and rail, move it towards the horizontal cabinet, reattach it, and screw it to the horizontal cabinet as instructed in the installation guide.

Leave the right-hand cabinet where it is. Buy a deco strip and cut it to fit the gap between the cabinet and the wall, or buy a piece of wood and paint it to match the cabinet. Also screw that cabinet to the horizontal cabinet.

I'd still recommend adding a cabinet to the left of the window.

Do you mean you couldn’t open the microwave door because the upper cabinet on the left was in the way of the hinge side of the microwave door? Your cabinets might be too deep for your microwave. If that’s the case, you may need a spacer behind the microwave to bring it forward, then you can move the cabinet to close the gap.

This one is from IKEA, I’m not sure what else is available.

https://www.ikea.com/us/en/p/nyttig-filler-piece-for-micro-hood-10282240/

You could choose some white tiles, resin, washable paint or a sheet of metal such as brushed stainless steel!

I didn’t see the front view photo before I posted. It looks like the front of the microwave is flush with the cabinets around it. In that case, you definitely need the filler or spacer behind the microwave to bring it forward so you can open the door. Then you can move the cabinet to the right and attach it to the cabinet over the microwave so there’s no gap. As for the gap between the cabinet and wall, you need a matching filler piece that you can purchase wherever you got the cabinets.

I'll bet he didn't mount the cabinets flush against the wall to avoid the window molding. That's why the microwave is mounted too deep and its door hits the adjacent cabinet.

use a filler strip

@Melissa R is correct, you could use a filler strip there. But that doesn’t solve the fact that the solution to your microwave door problem means the cabinet on the left is now installed on top of window trim.

Your microwave is too big for the space you have allotted for it, or you haven’t installed it properly. So pull down the cabinets and microwave, rehang the cabinets correctly, and move on from there.

I know it sucks to realize you’ve made a mistake, but scootching things around to cover it up just creates more issues down the road, as you now realize. Solve the actual original problem, the microwave, and everything will fall back into place.

BTW, you need a filler strip between the upper on the right and the wall as well. Good luck!

Sorry for the bad photo. Your microwave needs to stick out in front, like this. Here’s a photo of the spacer behind the microwave, too.

I think the comments that point out that the microwave needs to be mounted on filler strips would be the best solution. Then the cabinet could be mounted flush and it would not be on the window trim. Best look all around.

@parasol1@houssaon Thanks very much guys. I actually have filler strip with me, just had no idea how the wall exhaust vent touches the microwave vent outlet with the filler. Now they are touching properly without the filler.

Trim strip - paint to match (could be any piece of wood, just sand and paint to match). I'd suggest making it cover the whole gap - up top, down the front and underneath. I agree with cabinets on the left visually encroaching on the dining area - how about some of the white floating shelves to balance the cabinets on the other side, but still be "open" and not taking up the visual space of the dining area. Just don't get into the habit of cluttering them :-)