I don't believe there is any single official rate. Oanda, XE and WSJ are all good guides, but I don't think any of those show you the full range of rates on a given day. Since many currencies can have a trading range of well over 1% on a single volatile day, an average rate doesn't necessarily tell you if the rate you received was particularly good or bad. A Bloomberg terminal is probably the best source of data, since you can pull up a full chart of trading rates for any two currencies on any given day to make sure your transactions fall in the range. But most of us don't have one of those.

Also note that if you're looking at ATM transactions, you should be looking at the rates for the day of the withdrawal. If you're looking at credit card transactions, you should be looking at the rates for the transaction "posting date" listed on your statement and not the "sale" or "transaction" date.