Maybe a kinsman of Thingol, so he took command after the Sack of Doriath. Melian deserted the Sindar so it is possible that he could of been a cousin or nephew of Thingol. Only one question remains:What does the Silmarillion say about this curious character? Nothing. So kinsman of Thingol is my pick
Yes, The Return of the King notes Celeborn as a kinsman of Thingol.
So far anyway, Oropher seems to have been a relatively late character in Tolkien's imagination, appearing in one of Tolkien's 'late philological writings' for example, thus well after the 'Silmarillion' existed in various forms.
Oropher was Sindarin, and came from Doriath after its ruin, but had no desire to sail West nor to be merged with other Sindar of Beleriand 'dominated by the Noldorin Exiles'. So he with a 'handful' of other Sindarin Elves merged with the Silvan folk of Greenwood.
His movements, and reasons for moving, might depend upon which of the two 'Oropher texts' one chooses to deal with [is there another Oropherian source? I can't recall at the moment]:
A) a late philological writing, Appendix B to The History Of Galadriel And Celeborn in Unfinished Tales
B) note 14 to The Disaster Of The Gladden Fields, also Unfinished Tales
Both are late writings in any case.
In The Lord of the Rings as published [much earlier than these Oropher texts] it's implied that it was Thranduil who established a realm in the Second Age. Nothing is said of any 'Oropher' as founding a realm in Mirkwood, and readers of the books would, in my opinion, rather naturally assume that the realm met in The Hobbit is basically the realm Thranduil had established in the Second Age...
... except perhaps for Legolas' comment, for instance, in The Fellowship of The Ring [and maybe some other reason that I can't remember right now] where he suggests that the People of the Woods did not delve underground until the Shadow came, which might indicate that the underground fortress was not as old as the original realm.
First Edition wording: Tale of Years of the Second Age, Appendix B
'... many of the Sindar passed eastward and established realms in the forest far away. The chief of these were Thranduil in the North of Greenwood the Great, and Celeborn in the South of the forest.'
In the revised edition the remark about Celeborn was omitted, but Thranduil is still noted as king 'in the north' of Greenwood and being one of those Elves who migrated from Lindon before the building of the Dark Tower [Second Age] and established a realm in the forest.
So I think Tolkien's late texts about Oropher might not even have been in his mind when he revised The Lord of the Rings in the 1960s, but in any event, if JRRT was certainly going to go ahead with this more detailed history, he would maybe explain that the lack of any mention of Oropher was due to the notably compressed version of events given in Appendix B.
I'm not sure that's the way things were planned originally, but then again I can see Tolkien explaining that Thranduil did 'establish' his realm in the North in any case, ultimately, and especially the realm the reader of The Hobbit has already visited, even if there is more to this history than appears in the abbreviated account in Appendix B.
On the other, other hand however, the 'Oropher texts' were never published by JRRT himself, and they at least arguably contain conflicting elements, although I have stumbled across some seeming attempts to merge both accounts into one history. Anyway, anyone reading The Lord of The Rings might easily wonder...
... Oropher who?
What about the theory if him being a relative to Denethor, who perished in the First Age. Denethor's folk had never really seen the Noldo later. Hearing of their deeds of cruelty and malice, they became a little stiff to the brave but arrogant Sons of Feanor. When the Sack of Doriath came about Oropher might of disliked the Noldo even more so he did not want to be amongst or live with the friends of them.