Po2096 what o2 sensor do I need front or rear or both

What is your question?

Engine light came on the very first day I bought it at used car dealer, had the car only 3 weeks now, code po2096/poo31 showed up. I just need to know exactly what o2 sensor I need. Front or rear or both?

How long has your 2005 Subaru Legacy had this problem?

I bought it used 3weeks ago,same night I bought it engine light came on, I'm sure it was on before I bought it