Osorio Martínez was a magnate from the Province of León in the Empire of Alfonso VII. He served the emperor militarily throughout his long career, which peaked in 1138–41. Besides the documentary sources, which are somewhat meagre at times after his fall from royal favour, he is mentioned in two episodes in the Chronica Adefonsi imperatoris. He supported Ferdinand II of León after Alfonso's death, but he died in a Castilian civil war in 1160.
Osorio was the second son of Martín Flaínez of the Flagínez family and Sancha Fernández. In 1124 Osorio was granted the tenencia of Melgar de Abajo. In 1126 he joined the other noblemen of the Leonese province in swearing an oath of fealty to the new king Alfonso VII at the capital city ofLeón. Five years later he received Malgrat. The next year he received the tenencias of Riba Cetera and Villamayor, later also that of Ribera, but these he lost sometime between 1135 and 1137. As early as 1131 he was governing Mayorga and by 1135 the Liébana as well. In 1130 he and his brother Rodrigo were dispatched to besiege the rebel Pedro Díaz in a place called Valle. The Chronica Adefonsi records the campaign:
The King of León ordered Count Rodrigo Martinez and his brother Osorio to go to León. They were to attack Pedro Díaz who was rebelling in Valle. Díaz had with him a large number of knights and soldiers. Rodrigo and Osorio arrived there and encircled the castle. Those inside continued to shout insults at Rodrigo and his brother because they were unable to counterattack them successfully. When this was reported to Alfonso VII, he hurried to the town of Valle. He commanded that mantlets and numerous other war machines be constructed around the castle. The King's forces commenced to hurl rocks and arrows at those inside. Subsequently, the castle walls were completely destroyed.
In July 1138 Rodrigo died at the Siege of Coria and the king immediately appointed Osorio count in his place and granted his tenencias of León, Aguilar, :es:Tierra de Campos|Campos, and perhaps Zamora. Osorio and his brother's erstwhile vassals carried Rodrigo's body to the city of León for burial in the family mausoleum beside his parents. There they buried him in a church next to the Cathedral of Santa María, possibly the monastery of San Pedro de los Huertos, which his parents had received by a royal grant of Urraca of Zamora and Elvira of Toro in 1099. Osorio held numerous property interests in the city of León. Osorio's succession to his brother and the burial of Rodrigo are recorded in the Chronica Adefonsi:
Alfonso gathered all of his advisors, and in their presence, he appointed Osorio, Rodrigo's brother, to be consul in his place.... Count Osorio, the new consul, took the body of his brother to León. He was accompanied by his own military force and by that of his brother. The mourning over the death of Rodrigo Martínez increased in every city. In León they buried him with honors in his father's tomb near the Basilica of Saint Mary. The tomb is located very near the episcopal throne.