If you have a contractor working at your site, and they are found to have safety violations, OSHA can hold you responsible as the company that hired the contractor. It does not matter whether or not the contractor’s work puts your employees at risk. You still retain liability.
Screen contractors before hiring. Set up a process (like a check list or form) to gather all contractor data before signing the contract. That may include information about their work comp experience rating, a copy of their safety program, knowing who is responsible for overseeing safety on a daily basis, and insurance certificates.