Hah, what a coincidence!! I struggle the same with this looks and I too am busty! This morning I wanted to do my white jeans with a black&w graphic top and chunky white soled black loafers when felt I need a black shirt over it all. Kind of like a topper so wanted to leave it open.
I do have an older, kind of oversized (roomy and not tailored-but length on the hips, like yours here) black cotton button down but when tried on it was still not giving me the vibe...too short, too smallish...nothing to do with the vibe I wanted.
Then breefly tried on my trully oversized crisp white shirt (bought last summer-much LONGER-to mid thigh-...and also much roomier) and I was really pleased with what I saw in the mirror ref. fit and vibe-only it was a white.
Then wondered about the reasons.
In my mind both shirts were purchased as "oversized" but the black one was my first try with the silhouette years ago...and was totally lacking the drama of a trully oversized style- but beeing such a big departure from my usual silhouette back then I really thought this has it. Still, after some unsuccessfull trying ons I've given up on the silhouette and my shirt was left unused and forgotten. Thought it was the black color.Then last summer wanted to give myself another try with the oversized shirts, and bought me this time a white one, thinking is exactly the same cut like my black one...! Wow! Was I really wrong...
My revelation was that probably my eyes and brain needed more time to adjust to the new vibe and that maybe some of us really need more drama and exagerration for achieving the correct oversize look.
Long story short I think you could try a much longer shirt but I like the color of this one and the styling with the tweed gillet:-)). IMHO the knitted one doesn't do it any favour(like lisap said it so well).
ETA-I see after reading comments that my ideea was kind of the same to Approprio's. And, I know what others say about drape, too. But think, that is a completely another vibe vs. what I am reffering to.....for which I need length, width and crispiness.