Thanks to all for the advice. Come to find out that while I clicked on FRONT O2 sensor on the website, their computer had the part numbers confused and sent me the rear sensor, which I will keep because I am sure I will need it soon.
The subarupartsforyou.com site is, coincidentally enough, the same address as the dealer from whom I bought the car, so I will not be doing business with them. I live near another dealer who seems willing to match online prices for parts, so I will probably be dealing with them in the future.
Again, thanks so much for the part numbers, suggestions, etc.
Mike