We have no date of birth for Oropher, but he does seem to have been among the wave of Sindarin Elves who travelled eastward across Middle-earth before the year II 1000. He doesn't appear in any of the accounts of the First Age, which suggests (though does not prove) that he was born in the first millennium of the Second Age.

All our references to the Elvenking of Mirkwood are to Thranduil. Assuming the title to be hereditary, however, and further assuming that Oropher was indeed Thranduil's father, it is conceivable the Oropher was the original Elvenking. Following this line of reasoning, Thranduil would have inherited the title at the time of Oropher's death in the Battle of Dagorlad.