Master of the Osservanza Triptych

The Master of the Osservanza Triptych, also known as the Osservanza Master and as the Master of Osservanza, is the name given to an Italian painter of the Sienese School active about 1430 to 1450.
The Italian scholar Roberto Longhi recognized that two triptychs formerly attributed to Stefano di Giovanni were the work of another hand, now generally referred to as the Master of the Osservanza Triptych. The Virgin and Child with St. Jerome and St. Ambrose and the Birth of the Virgin are both stylistically similar to the work of Stefano di Giovanni, but have a narrative expression that is characteristic of Late Gothic painting.
Longhi observed that another group of paintings was closely related to these works and appeared to be by the same hand. These included the predella of the Osservanza Altarpiece, a predella of St. Bartholomew, Scenes of the Passion, the Resurrection, and Scenes from the Life of St. Anthony Abbot. Additionally, the full-length painting of St. Anthony Abbot in the Louvre appears to be from another altarpiece by the same master.
Research in 2010 by Maria Falcone in Siena has revealed the name of the Master to be Sano di Pietro.