After the Baroque period, the Classical period brought a regular oboe whose bore was gradually narrowed, and the instrument became outfitted with several keys, among them were those for the notes D♯, F, and G♯. A key similar to the modern octave key was also added called the "slur key", though it was at first used more like the "flick" keys on the modern German bassoon. Only later did French instrument makers redesign the octave key to be used in the manner of the modern key (i.e. held open for the upper register, closed for the lower). The narrower bore allows the higher notes to be more easily played, and composers began to more often utilize the oboe's upper register in their works. Because of this, the oboe's tessitura in the Classical era was somewhat broader than that found in Baroque works. The range for the Classical oboe extends from C4 to F6 (using the scientific pitch notation system), though some German and Austrian oboes are capable of playing one half-step lower.
Taken from https://en.wikipedia.org/wiki/Oboe