The technique of the oboe family is generally quite refined and flexible. The only part of the instrument that offers up some resistance is the very bottom register. Most of the notes in the bottom third are controlled by the little fingers (like most woodwinds), and quick transition (i.e. trills) are simply not possible. The only truly impossible trill is from the low B-flat to the low B on the Oboe. In fact, remember that the low B-flat is only available to the Oboe proper. Extensions to B-flat have been developed for the other members, but are additions to already existing instruments and their use generally means that the low B-natural is no longer available. As a general rule, just avoid the low B-flat for the lower oboes altogether. While the lowest notes of the lower oboes are beautiful and sonorous, the low B and B-flat of the Oboe are raucous and loud. Most Oboists I have spoken with feel that their use should be limited.
One addendum to the lower range discussion is that the Loreé company produces an Oboe with an extension to low A. This note, as far as I can tell, has never been used by composers, though a few oboists will use this note in transcriptions.
Various texts I have read claim different upper limits for the oboe’s range, and very few are correct. All members of the oboe family (with the possible exception of the Piccolo Oboe) ascend to the F above the treble clef. This range can comfortably be extended to a G by most Oboists (but is best avoided on the lower instruments). I once attended a concert for double reed players by double reed players, and the crowd was in utter astonishment when one of the best Oboists in the world played a piece that ascended to an A above this G. Yet, we find this A in works of Stravinsky and Dvorák. Most Oboists I have talked with simply omit the passage or take the offending part down an octave. Don’t write above G. Oboists carry sharp knives with them; I certainly don’t want to offend them with my writing. This said, I personally once wrote an Oboe concerto that included a note (never mind you which one!) that was above this G, but I did it with the full cooperation of the player involved, and his production of this particular note (though not of the other notes in this register) was secure and sound. Notes in the extreme high register lose their characteristic sound, and can be quite painful to listen to (and to produce).