I don't think that Oz showing up was vital, i think all he did was to help push the relationship along. Oz made Willow realise how much Tara means to her, and that she was the most important thing in her life and wasn't going to let her go. But she would have realised that eventually, it's just oz made her realise all those things all at once, and very strongly
I just want to say that life is entirely too short. Carpe Diem is the word. You gotta do what's in your heart. You gotta live your dreams." ~Amber Benson