Do d.o.'s do neck cracking or neck adjustments like chiropractors do (for neck tightness)?

Possibly: D.O.'s are physicians who are also trained to perform manipulation of the spine, as well as other joints. Interview your local D.O. To see if they do manipulation as part of their practice.

D.O.s do adjustments: Doctors of osteopathic medicine, or D.O.S, are trained to diagnose and treat musculoskeletal issues such as neck and back pain with omt(osteopathic manipulative treatment). Omt provides many options for treating diverse medical conditions and is similar to chiropractic in some ways, but also unique and can vary depending on the physician. Dos also have equivalent education and training to mds.

Somewhat: D.O.'s have been trained in adjustment therapy but have the significant advantage of understanding diseases of the body and spine in the light of modern medical knowledge. A chiro is likely to strenuously massage the neck without a diagnosis and possibly cause permanent damage. A D.O. Will given gentle and focused neck massage that is likely to be helpful and not dangerous.