Yes osteoarthritis is sometimes a serious disease for some people.
For some people in the early stages of osteoarthritis the disease is not as serious or very crippling but as the osteoarthritis disease progresses it can become more serious.
As the person ages and experiences the osteoarthritis it can become more crippling.
So yes the osteoarthritis disease is considered to be a serious disease by most doctors.
The osteoarthritis typically only affects older people in their 60s and 70s and 80s and above but anyone can get affected with osteoarthritis.
Treatments can help with the osteoarthritis disease but as of now there are no cures for osteoarthritis.
Once you have osteoarthritis you'll have the osteoarthritis disease for the rest of your life.