Just found out we can close on the house I am buying on the 30th. I called landlord to inform him of my intent to move. He said I had to give a 30 day notice. I have been living there 3 years, the past two years with no lease.
He also said if I did not give him a 30 day notice it would affect my credit.
Under Ohio law, if there is no lease, just a presumed month to month do I have to give a 30 day notice?

You have to give the same notice as the term under which you pay rent. If you pay rent once a month you have to give at least one month's notice. For example, if you intend to move before July 1st you must give WRITTEN NOTICE before June 1st - one month in advance.

You don't have a "presumed" month to month tenancy. You DO have a month-to-month tenancy.

Yes, if the landlord sues you and gets a Judgment it will affect your credit.

(The length of time you have lived there is immaterial.)

Judy is, of course, correct. See Lawriter - ORC - 5321.17 Termination of tenancy.

Why would you think you don't have to give your landlord proper notice? Isn't he entitled to time to get a new tenant? Should he lose rental because you want to move?

Not giving proper notice will not affect your credit. However, if you don't pay the rent you owe, he would have to file suit which could affect your credit.

None of this will prevent you from closing on the 30th. But you will owe your landlord the rental for July. And he is legally entitled to it.