I have exactly the same situation,, except I can't blame the previous owner

When I remodeled a bathroom last summer, The big shower stall my wiff wanted was too big to put where the old corner shower was, so the only way to make it work was to put it along a wall that had a octogon shaped window,  [actually not a window , but a feature of the old bathroom ] so I cut a piece of plywood to fill iit in just as they did,[ notice I said THEY and not YOU] I fully intended to paint the ply wood, but didn't  but it has a wooden trim around it that I can take off and replace the wood with a stained glass window like I had planned to do just ain't done it yet, probably never will,

I don't know what could be done about your problem,, is the wood covering something on the back side? shutters would be a great idea,, but then you would have to put shutters on the rest of the windows, to be consistant,,

OR!!!  you could hire a artist to paint a mural of a woman and a man taking a shower or a kid throwing a hissy fit or a couple fighting, or just a simple old ugly dude staring out the window,, so it would look real,, or leave it alone and don't look at it anymore,, and it will go away,, maybe

Directly behind that plywood is sheetrock, texture and a large mirror.  At first I took the mirror down in the bathroom, thinking that I could put a curtain in the window and be done but there was no window, just a sheetrocked and painted walll