Osmand moves to the Hoops today in a free transfer as he comes off contract with the Cottagers.
Fulham offered Osmand a new deal, but he chose to turn it down.
Osmand, who joined Fulham at the age of 13, scored 18 goals for the U21s side last season.
The 19 year-old has signed a deal with Celtic to 2029.
Celtic manager Brendan Rodgers said: “We are really pleased to bring Callum to Celtic. He is a highly-rated, talented young player.
“He has some real attributes. He’s a quick and powerful striker and I’m really looking forward to working with him. I’m sure he can be really successful with us.”
Unbelievable to be here
Speaking to Celtic TV, Osmand said: “It feels unbelievable to be here and I am so happy to be here. It is a big step for me, and I am excited to get going.
"It’s a massive move and I can’t wait to step out at Celtic Park in front of all the fans.
“When I spoke to the manager, he was really positive. The plan and pathway he has set out for the club is top, so it’s exciting with the pathway he has for me.
“He has set out what he wants from me, and it is all really positive, so I can’t wait to get going now.
“I’ve also spoken to Matt O’Riley on the phone, so he has given me advice. He was telling me about how big the club is, how amazing the fans are and it’s a new life really. So excited is definitely the word for me.”