Can a chiropractor or a medical doctor fix chronic lumbago?

Depends. A chiropractor will offer a deep massage which might temporarily give some relief. You need a medical doctor, preferably a physiatrist or orthopod, to actually diagnosis your problem and recommend treatment likely to cure your lumbago.
OMM. Low back pain is one of the areas where osteopathic manipulation has been shown to be particularly effective. Osteopathic medicine includes the same medical training as an md, combined with manipulative treatments similar to (but distinct from) chiropractic care. In this way, your local do is able to use the widest possible variety of modalities to treat your low back pain.
It depends. The natural history of lbp is to resolve, but depending on the underlying diagnosis, your pain can be alleviated.