Do you have to officially register it with the US Copyright Office in order for it to be considered copyrighted?

Answer 1

No.

Registration of copyright is voluntary. Your work is protected as soon as it is "fixed in a tangible form, recognizable by human or machine".

However you will have to be registered if you wish to bring a lawsuit for infringement in US courts.

Answer 2

Yes; works of sufficient originality are automatically protected as soon as they are fixed in a tangible medium.

However, works that were published in the USA prior to 1989 were required to follow additional rules or they would NOT be copyrighted in the USA.

For example, the had to have proper "copyright notice" in order to be copyrighted; things published prior to 1964 were also required to have a "renewal" filed at 28 years after publication. For these types of works, if there were inadequate notice, renewal or registration, then those works are NOT copyrighted.

Copyrights of certain foreign works that had not lapsed in their original countries were "restored" in the 1990s, even if they had never complied with US copyright requirements (such as notice, renewal or registration).

Answer 3

The Copyright Office accepts registration for all copyrightable works, however, registration is not required for protection. For works copyrighted in the USA prior to 1964, the renewal of the copyright would be filed in the Copyright Office, or else the work enters the public domain 28 years after the date of publication.

Transfers of copyright ownership may also be recorded in the Copyright Office.

There are many other types of information "registered" in the Copyright Office, which is a branch of the Library of Congress, as required by their regulations.