Dress code
Black tie optional means a dinner jacket if you have one, and a dark, sharp suit if you do not.
Black tie optional means a dinner jacket if you have one, and a dark, sharp suit if you do not.
Black tie optional gives you the choice: wear black tie, or wear the most formal suit you own. Both are correct, so nobody is out of place.
The trap is the middle. A suit that is nearly a tuxedo but not quite looks like a mistake. Either commit to the dinner jacket, or wear a clean dark suit with a white shirt and a quiet tie.
Women have the wider range here. A floor-length gown, a long dress or a tailored suit all work, because the code is about the level of polish, not one uniform.
The lever · Whichever route you take, let the shoes and the tie agree in tone; that single match reads as deliberate.