The 21-year-old is yet another option in manager Eddie Howe's squad ahead of the upcoming season as players attempt to earn their place in the squad.
Osula spoke about how happy he is to sign for the club and what an opportunity it is for him.
"I'm very happy to be here. It's a big club, a great club, so I'm very happy for the opportunity to join Newcastle United.
"It's a great opportunity, and as soon as I heard about Newcastle's interest, I knew I had to take the chance to join this club if it came. It has come, and I'm really happy to be here."
Howe spoke about his talent and why he opted to sign the forward.
"William is a talented young forward with all the attributes to be an exciting player for Newcastle United.
"We have watched his development closely and we are delighted to have the opportunity to work with him in this next step in his career.
"I know our supporters will join me in giving William a very warm welcome to the club and the city."