A proper Oxford shirt is the one article of clothing I tend to wear year-round, regardless of the weather I'm experiencing or the country I'm visiting. Perhaps the best thing women borrowed from the wardrobes of working men, Oxfords carry me from the dead of winter (under a thick sweater, of course) right through to the hottest moments of summer.
Worn loosely buttoned - short pants or skirts are optional, if you sport a larger, shirt-dress style - the better Oxfords offer a pulled-together look without sacrificing breathability in sweltering temps. I prefer the washed blues, not unlike Acne's Lord Snowdon shirts, or this soft-to-the-skin Land's End version.
The trick is to always get one with a sharp-ish collar and firm placket for the buttons, as those are the main design elements that will keep your Oxford looking polished, even if you're wearing it all crumpled-up as a cover for the odd beach vacay.
Which is exactly what I tend to do with mine...
NB Images Courtesy Land's End Canvas