We find the "scary, handsome genius" at his seemingly most vulnerable: blind with only a Terminator-esque visual display via his sonic glasses giving him limited sight at best.
We're used to the Doctor overcoming improbable odds, but his disability creates an added dimension of drama, quite literally in this episode of course. Fortunately the Doctor is still able to interrupt a companion's date thanks to an impromptu materialisation in Bill's bedroom, much like gatecrashing Clara's restaurant date with Danny.
Given the Doctor's affliction, there's more tension to the Monks' gradual reveal through his eyes (never looked so good in the morning, indeed). It also takes on a metaphorical meaning when Cardinal Angelo offers the Doctor the opportunity to confess, which makes him lost in reverie to when Nardole, on River Song's behalf, intervenes just before Missy gets executed. Nardole's arse-kicking threat to the Doctor essentially translates as: never lose sight of who you really are, Doctor.
Capaldi, himself, described the challenges of playing the blind Doctor in terms of "not making eye contact" and "not giving the actors what you felt they might need".
In creating such a bold and dramatic narrative twist at the end of Oxygen, some felt it was a retrograde step when he regained his sight with a more or less click of the fingers from Bill in The Pyramid at the End of the World.
However had that not been the case, the Doctor’s out loud thoughts of "maybe my future regenerations will be blind" would’ve given it more of an intriguing impact, particularly knowing that Capaldi was leaving the show.