Huw Clement Evans and Christopher Cowie explain some of the possible effects created by the oboe and cor anglais.
A close up of a cor anglais reed. Vibrato is a slight undulation in pitch. It can also be a variation in intensity of sound. Either way the effect is of a sound that 'wobbles' slightly. It is a written instruction in the music - Vib. or Vibrato. Sometimes with a wavy line just after the note. Vibrato is integral to oboe and cor anglais technique, but can also be specifically notated as a special effect. For example, if a composer wants lots of vibrato, they may write Molto Vib. or if a plain sound is required, Non Vib.
A slide in pitch between notes. The beginning and end note of the glissando are written and connected by either a straight or a wavy line. Usually the word gliss or glissando will be written above. Glissandos are more or less impossible on the oboe. A few ascending glissandos may be possible on the cor anglais (not descending), but this is unusual.
Description: a soft object inserted into the end of the oboe to muffle the sound. Notation - con sord. Mutes for oboe and cor anglais are usually homemade, improvised items.
Tremolo or "shake"
A trill or rapid alternation between two notes more than a tone apart. Notated by thick slanting lines between two notes. The tremolo shown on the left lasts for one beat. The more fingers that need to be moved the more unreliable the effect becomes. This is especially true when using high notes.
A rapid alternation between two pitches - either a tone or a semitone apart. Notated by accidentals over the note. If these are not given, players will choose a trill type based on musical context. Wavy lines to indicate the duration of a trill are optional. On both the oboe and the cor anglais trills do not present a problem in the lower two octaves but higher up the awkward fingerings can make trills very difficult.