Ostoher was the son of Anardil and descendant of Anárion, who ruled Gondor as its seventh King. Early in his reign, he undertook a great construction work, the rebuilding and expansion of the Minas Anor. Reconstructing the citadel was a considerable task (some sources suggest that it took as long as ten years), and it seems to be the source of Ostoher's name (which means 'fortress lord'). After the rebuilding, Ostoher took Minas Anor as his summer residence, and this tradition was carried on by his successors.
Much later in his reign, Gondor suffered its first serious assault since the defeat of Sauron some five centuries before. Bands of Wild Men appeared out of the East, threatening Gondor's eastern lands. By this time, Ostoher was in his old age, nearly 270 years old, and it fell to his heir Tarostar to meet the Easterling threat. He succeeded in driving back his enemies, but Ostoher died before his son's victory was complete.
Ostoher ruled Gondor for eighty-one years, and was succeeded by his son, Tarostar, who became known as Rómendacil I.