Do you need cdl in Ohio to drive semi for farmer?

If it's a registered farm vehicle, with farm tags, and it is only used in support of the operations of that farm, and you stay within the distance you're permitted to drive from that farm, and you're a direct employee of that farmer, then no, you're not required to have a CDL. Registered farm vehicles operating under those conditions are exempt from CDL requirements. However, the state of Ohio has jurisdiction for licencing requirements at that point, and you need to check with the DMV to see what their non-CDL licencing requirements are for an exempt tractor-trailer. They might have a system in place where they offer non-CDL equivalents to Class A and B CDLs, or they might have a "chauffeur licence" system with endorsements for tractor-trailers.