Hi Catherine, Generally there is nothing special other than the usual registration on the Secretary of State with Articles of Organization https://www.ohiosos.gov/businesses/ Check the last box about Revised LLC Act as of Feb. 2022. For the clients this might help https://ohio.gov/business/resources/business-first-stop
SMLLC is just Sch. C and no special filing as Ohio income tax begins with federal AGI. There is a 'big' exclusion of business income for taxes up to $25,000, though. If MMLLC, then 1065 and, as a pass-through entity, there may be additional requirements for out of state members. It can get a bit complicated.
So this is a start and follow up depends on what the clients decide to do. That, of course, will lead to the right research path. Good luck!
Oh, just be aware of the multiple municipalities that all want to tax so the location of their business is important to know.