§ Osgiliath >> Elostirion, Ostirion ’= fort’ (TI:423)
This changed name of Osgiliath ’fortress of the stars’ (Let:347) looks like a translation into Quenya with el- ’star’ (EL-) + ostirion < osto ’city, town with wall around’ (OS-) + tirion ’watch-tower, tower’ (TIR-), translated as ’fort’.
Note that in The Silmarillion, Elostirion is a tower upon Emyn Beraid (western Arnor), where one of the palantíri is kept in the Second Age.