The Marshall County Sheriff's Department said a third worker was hospitalized, KWTV in Oklahoma City reported.
KTEN-TV in Texoma, Texas, said the explosion occurred about 4 p.m. in a furnace at the plant, which is on the outskirts of Madill.
Investigators had not determined the cause late Monday, KWTV reported.
The Oklahoma explosion occurred about 6 hours after an explosion caused a collapse at an International Nutrition feed plant in Omaha, killing at least two people and injuring several others.
Notable deaths of 2014 [PHOTOS]