The oboe d’amore is the alto member of the oboe family – although when the Cor Anglais is included as a fully paid up member of the family, which it often is, then the oboe d’amore can be considered as the mezzo to the oboe’s soprano and the cor anglais’ contralto (alto). It was common in baroque music - J. S. Bach used it extensively in his cantatas and particularly in the St. Matthew Passion. Bach’s Harpsichord Concerto, BWV1055, is thought to have been originally for oboe d’amore. In modern times Richard Strauss gave it a part in his “Symphonia Domestica” and it has a turn in the ubiquitous “Bolero” of Ravel.
It has a deeper, softer, more mellow sound than the modern oboe and its presence is missed in the woodwind sections of modern orchestras attempting certain pieces of baroque music using modern instruments. The proliferation of ensembles playing pre-Classical music using "original instruments" is bringing the oboe d'amore, and others, out of obscurity.