What OSHA actually requires
There is no such thing as an OSHA-certified rigger. That surprises people, and it changes what you should be asking for.
OSHA does not run a rigger certification program and does not accredit anyone else's. What the regulations do is put the decision on you, the employer, and then expect you to be able to defend it.
Which rules apply to you depends on where you work. Plants and warehouses fall under 29 CFR Part 1910 (general industry). Construction sites fall under Part 1926. The requirements below are labeled accordingly, because applying the wrong one is its own problem.
Sling inspection, general industry: a "designated" person
OSHA 1910.184(d) requires slings to be inspected each day before use "by a competent person designated by the employer." The standard defines designated as "selected or assigned by the employer or the employer's representative as being qualified to perform specific duties." It does not define competent person, and it sets no training syllabus, no exam and no certification card. You decide who is qualified.
It does require one piece of paper: under 1910.184(e)(3)(ii) you must keep a record of the most recent month in which each alloy steel chain sling was thoroughly inspected, and produce it on request.
Sling inspection, construction: before each shift
On construction work the equivalent rule is 1926.251(a)(1): rigging equipment for material handling is inspected prior to use on each shift and as necessary during use. The qualified rigger requirements below come from the construction crane standard and do not reach a general-industry plant at all.
Rigging, construction: a "qualified" rigger
The construction crane standard requires a qualified rigger for specific work: rigging during assembly and disassembly (1926.1404(r)(1)), and rigging where employees are in the fall zone hooking, unhooking or guiding a load (1926.1425(c)(3)). A qualified person is defined at 1926.1401 as one who, by possession of a recognized degree, certificate or professional standing, or by extensive knowledge, training and experience, has successfully demonstrated the ability to solve or resolve problems relating to the subject matter, the work or the project.
There is still no mandated certificate. OSHA states that employers determine rigger qualification against the hoisting job to be performed, and that an employer may use a third party to assess a rigger candidate but is not required to.
So the question an auditor asks is not "is he certified?" It is "on what basis did you designate this person?" Documented training is the most common answer, and it is the one we help you build.
The exception: signal persons. Of the subjects we teach, signal person qualification is the one place OSHA is prescriptive. 29 CFR 1926.1428 requires documented qualification through an oral or written test and a practical test, carried out either by a third-party qualified evaluator or by your own qualified evaluator. An assessment by your own evaluator is not portable, so another employer cannot rely on it. Our signal awareness session does not qualify a signal person, and we will not tell you it does. The documentation also has to be available at the site while that signal person is employed. If you need qualified signal persons, you need an evaluator who runs those tests.