Oropher was a Sindarin lord of Elves. He was one of the Sindar of Doriath, but after the War of Wrath he declined to depart Middle-earth as many others did, and instead went over the Ered Luin with his house-hold. He eventually ended up in Greenwood the Great, where Silvan Elves of Nandorin descent lived, and he was taken by them as lord. His capital was at Amon Lanc.
West of Oropher's realm was the realm of Lórinand across Anduin, where Amdír, another Sindar ruled over Silvan Elves. When Sauron returned to Middle-earth and southern Greenwood became dangerous and was renamed Mirkwood, Oropher's folk retreated north of the old Dwarf Road, and later again north of the Mountains of Mirkwood, where they fortified themselves.
Oropher answered the summons for the Last Alliance of Elves and Men, and joined with Gil-galad's forces as they marched down Anduin to Dagorlad. Oropher's army was lightly armed. In the battle of Dagorlad Oropher's company fought valiantly, but he was slain with the greater part of his people when he agrew hasty and called an early charge.
After Sauron was defeated, Thranduil, Oropher's son and heir, returned with the remainder of his people north back to Mirkwood. While one third of them survived, their army was still large enough that the Orcs hiding in secret in the Hithaeglir did not dare attack them.