No they are usually not at the same time. I had my test on the 27th of July, decision made on the 28th of July and the oath invitation will take between 3 and 5 months, at least this is what we were told during the test. Very likely it is less than that.
It could also mean that they have decided NOT to let you get your citizenship, but I assume they would have advised you during the interview to withdraw the application if they saw any major mistakes.