"He knew something was wrong for a while," a source told Us Weekly. "Everyone is sad, but he is being strong about it."
On Monday morning, Osbourne's top tweet was one of hope.
"Thank you all so much for the kind and inspirational words," he wrote. "It means a lot. #adaptandovercome."
Osbourne's mother, Sharon, was by his side when he shared the news with People.
The 26-year-old has been a bit of a turnaround king of sorts in recent years, shedding a significant amount of weight and beating a drug addiction.
Sharon even attributes her son's 2003 sobriety to helping her famous rocker husband get off the bad s**t.
"I was just angry and frustrated and kept thinking, 'Why now?' " Jack said. "I've got a family and that's what's supposed to be the most important thing."
The Mayo Clinic reports that, with MS, the body's immune system attacks itself -- eating away at the sheath that covers the nerves, and disrupting communication between the brain and the rest of the body.
Symptoms and progression of the diseases varies dramatically, it reports, but in some severe cases people with MS may lose their ability to walk or speak.