Clarinets are famed for being able to play diminuendo and crescendo to and from niente. Oboes, however, have trouble playing in very low dynamics. The difference in construction of oboe and clarinet reeds/mouthpieces is obvious. What are the underlying processes behind sound production in the reed and mouthpiece that could explain this difference?
In this admittedly limited study, they record one oboist using more than double (over 110 cm H2O) the blowing pressure to play fortissimo compared to two different clarinetists (both around 50 cm H2O), also playing fortissimo. The other oboist in the study blew a peak pressure of about 80 cm H2O for fortissimo playing. A better graphical comparison is available here.
Both oboists used a pressure of about 50 cm H2O to blow pianissimo, which is about the same pressure the clarinetists are using for fortissimo! This suggests that playing the oboe requires a lot more pressure than playing the clarinet for the same dynamic level.
The main gap in the links above is a relationship between blowing pressure and measured sound pressure level. Fortissimo and pianissimo are subjective musical concepts. We would really like to have the minimum measured output level from both a clarinet and an oboe to see how quietly they can be played.
Strangely, this study records a range in output level of about 71 - 100 dB SPL for several oboists blowing G4, while this study shows a range of 80 - 110 dB SPL for the same note on the clarinet. In light of this information, we should remember that we cannot compare sound pressure levels to establish what is considered pianissimo on different instruments, because many other factors affect the perception of the dynamic level of an instrument, probably most importantly the formant (an aspect of timbre).
The oboe has a conical bore, compared with a closed cylindrical bore of the clarinet. That means the oboe will have stronger even harmonics than the clarinet. From the venerable Benade (chapter 13), we recall that increasing the presence of upper harmonics often makes a sound at the same intensity (SPL) sound louder.
So where does that leave us?
- It takes more pressure to play the oboe versus the clarinet.
- For the same loudness, an oboe is putting out a lower sound pressure level than a clarinet, which suggests the formant of the oboe "sounds louder".
Combining those two facts, the brave oboist must put considerable effort into making even the quietest of notes (although still less effort than a trumpeter!), and that note may still sound louder than a clarinetist can blow with less effort, due to the particular timbre of the oboe versus the clarinet.
And this still does not address the general difficulty of establishing effective embrouchure on a double reed instrument versus a single reed instrument, as pointed out by Carl Witthoft in his comment.