not sure what you want to know exactly, but my first thought is hanging REAL oars on the wall for art instead! Maybe mixed in with photographs, or hanging pictures of oars from an actual oar that is hanging horizontally....guess we need a picture of what you are wanting to do and where you are wanting to do this! :)

Many of these answers seem so firm. But each and every house is different, each and every sideboard is different and each and every mirror is different. The main thought is how to balance the width of the sideboard by considering wall it sits against. Then you must consider the height of the wall to achieve balance, by combining the height of the sideboard, the height of the mirror, then the height of the wall. You cannot change the height of the wall or the mirror or the sideboard without replacing them. So that leaves you to balance out the look of the three to best advantage when combined. In my home, the wall where my sideboard sits is not only tall, it sits over 20' from the entry. So putting my 7' high mirror, 12" above the 36" sideboard, was the height for an overall balanced position from the entry, And is usually considered the largest gap to be allowed between the two pieces according to interior decorators. I have a friend, who placed her mirror directly on her sideboard due to the low walls in her home and it looks great. Its the overall effect that matters.

Weirdness at the "function junction" of the ceiling heights between the kitchen area and the rest of the space complicates things. 1.) ceiling color (is the boxed part of the kitchen the same color as the rest of the ceiling?) I can't tell where the ceiling in the kitchen ends and the ceiling in the ajoining room begins. I think that's the key to solving your problem. The ceiling has to be clearly defined. 2.) I am a fan of bringing window treatments to the ceiling. But only if have a clearly defined ceiling! (you don't). I am also big on using valences to define the open spaces and conceal the "workings" of window treatments. Maybe a discreet valence would help you marry the "function junction" more easily? I love the neat, clean look of pinch pleated drapery panels mounted at the ceiling. But you have to have a clean ceiling to pull the look off with aplomb. (see alisa4evr's picture)

Wow Lots of comments. Way to go on the post! It really has drawn attention. This is my suggestion #1 due to the laws of odd numbers as mentioned above. The height would depend on how you use the space if you are mainly sitting down then I like the idea of the art being lower. As you already know from the gallery experience to get visually lost in an artwork your eyes should hit 3/4 the way up the piece. The addition of a third piece could be another illustration by the artist or an artistic representation of the artists name.