Several recent questions have been about tuning. Often an orchestra will tune to the oboe. One reason for this is that oboes are the only instruments that cannot be tuned. 1. What if the oboe was actually out of tune for some reason. 2. When the oboe is not present, who takes over? 3. For piano concerti,would the piano take precedence? And why would all this happen?
Oboes most certainly can be tuned (perhaps less tuning range than other instruments).
If there's a piano (or even worse, an organ :-) ) involved, typically the oboist will tune to the piano first, then let the orchestra tune to the oboe.
String orchestras typically tune to the first violin; chamber ensembles work it out one way or the other. FWIW wind ensembles (aka wind orchestra, concert band) tune to the first clarinet instead of the oboe. Damn if I know why. (Cue the first song from Fiddler on the Roof)
As to why the oboe... well, who wants to anger those who play "the ill wind that nobody blows good" ?
Carl's answer is right: you can tune an oboe and if a fixed tuning instrument (like a piano, an organ, a piano accordion, etc.) is part of the ensemble then the musicians will tune to that.
I had always understood that the oboe was used because of the harmonic composition of its timbre, i.e. that it has a relatively pure pitch and is thus easier to tune to. However Wikipedia's page on the oboe suggests that it is the oboe's secure pitch and penetrating sound, resulting from its conical bore, that make it ideal for tuning purposes.