The bass or baritone oboe is a double reed instrument in the woodwind family. It is about twice the size of a regular (soprano) oboe and sounds an octave lower. It has a deep, full tone somewhat akin to that of its higher-pitched cousin, the cor anglais. The bass oboe is notated in the treble clef, sounding one octave lower than written. Its lowest note is B2 (in scientific pitch notation), one octave and a semitone below middle C, although an extension may be inserted between the lower joint and bell of the instrument in order to produce a low B♭2. The instrument's bocal or crook first curves away from and then toward the player (unlike the bocal/crook of the cor anglais and oboe d'amore), looking rather like a flattened metal question mark. A another crook design resembles the shape of a bass clarinet neckpiece. The bass oboe uses its own double reed, similar to but larger than that of the cor anglais.

Some confusion exists between the bass oboe and the heckelphone, a double reed instrument of similar register introduced in 1904, and which is distinguished from standard members of the oboe family by its wider bore, different fingering system (on older instruments) and larger bell. As a result, it is not always clear in English orchestral works of the early 20th century which of the two instruments is intended when the composer requests bass oboe.