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 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.
The date of Ostoher's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
The 'fortress' referred to in Ostoher's name must surely be Minas Anor, which he greatly enlarged and took as his summer residence. That implies that Ostoher was not this King's original name, but a royal title taken to commemorate his most notable achievement.