I don't get it, how did the sulfur get an oxygen, I'm unable to find any any half reaction wherein a oxygen ion is generated. So what is happening here?
You can oxidize sulfides just by roasting them in the air, there is no solution, no ions there at all. Why do you think you need "oxygen ions" for anything?
But why is acidic medium required there then?
What do you know about solubility of sulfides? Can you think about a way of shifting the dissolution to the right?
What about protonating S2-?