The Oath of Allegiance is considered to be the last stage of the procedure for you to get US citizenship.
After Form N-400 known as Application for Naturalization is approved by USCIS, your next step is to take part in the ceremony where you need to take the Oath of Allegiance. Factually, you can not become a citizen of the USA without taking it. The ceremony hosts will ask you to give them your tree card in exchange for a Certificate of Naturalization. This document confirms your official US citizenship. As soon as you get it, your citizenship status must be updated in the Social Security record and then you can ask for a passport in the U.S. Department of State.