On his left lapel.
I’d have to see a photo of the office feom that angle to determine if it was left out on purpose.
So goes America, so goes FreeRepublic.
American flag is behind his right shoulder, out of view in this perspective.
https://duckduckgo.com/?q=Oval+office+and+the+US+Flag&t=chromentp&atb=v257-1&iar=images&iax=images&ia=images