Oscar F. Mossberg came to the Unites States in 1886 at age 20 from Sweden. He worked for a boiler shop for eight years before joining the Iver Johnson Arms and Cycle Works in Fitchberg, MA. where he invented the hammer block to prevent accidental discharge of the revolver if the hammer were struck in the down position.  This development led to the “Hammer the Hammer” slogan Iver Johnson used for generations.

After three years with Iver Johnson, Mossberg moved to Shattuck Arms Co. of Hatfield, MA. where he served as superintendent, overseeing the making of single and double barrel breechloading shotguns. Then in 1902 Mossberg joined the J. Stevens Arms and Tool Co. in Chicopee Falls, MA where he stayed for 13 years.

In 1919, helped by his sons Iver and Harold, he started the O.F. Mossberg & Sons, Inc. as a partnership in his barn. Even the partnership idea was new.