How the Oboe is Made
The sound is adjusted based on the shape of the tone hole?

The photograph depicts the round-ended key posts and rods being affixed after the tone holes have been drilled. Although the tone holes take many forms, overall, they tend to be small. The smallest tone hole has a diameter of only 2 mm. These small tone holes are characteristic of instruments such as the oboe and the bassoon.

Sound pressure builds up inside the pipe when sounds are produced. If the tone holes were as large as those on a flute or saxophone, there would be plenty of escape routes for the sound pressure, so the sound pressure would completely dissipate as soon as flow reached an open pad. Because the oboe's tone holes are small, however, the effective length of the sound waves stretches all the way to the end of the instrument. This somewhat irregular aspect of the oboe gives it its unique tone.

In addition to adding chambering, the construction of tone holes includes shaving the spots where the funnel of the hole and the bore of the pipe connect. This is called undercutting.
How the undercutting is shaved affects the timbre. The more is shaved away, the higher-pitched and brighter the tone. However, if even 0.1 mm too much is shaved off, there is a danger that the sound will be flat and hollow.

An oboe's timbre and pitch would still be unstable if holes were merely added. The depth of the chambering and the amount of undercutting adjusts the clarity of each note and the resistance felt by the player when playing, making the music sound smoother when scales and melodies are played.