Well, a couple of certifications that would be helpful would be Occupational Hearing Conservation (Council for Accreditation in Occupational Hearing Conservation), a NIOSH Spirometry course, a course in urine drug screen collections and breath alcohol testing that follow the DOT guidelines.
As the RN, you may or may not need them depending on the type of facility you work in. If it's a small clinic, you may have to do some of everything. If it's big with lots of other workers (MA's, x-ray techs, etc.) there may not be a need for you to do these.
Personally, it can only help you. Knowing the basics will you help you in the long run. These classes are not cheap or short! LOL.
You cannot get your COHN/COHN-S until you've worked full time for about 2 1/2 years in OHN. Same with the case management portion.