What's a good locale to have a graduation party in Cincinnati?

It depends on what kind of party you want to have and how large of an event you are planning. There are many different facilities you can rent out, such as the 20th Century Theater in Oakley. A number of restaurants also have large rooms that you can reserve for groups, such as the Vault at Teller's in Hyde Park. Teller's used to be a bank and the Vault room is actually the vault from the old bank; it seats about 25 guests. Another option is to have your party at one of the Cincinnati or Hamilton County parks. There are many lovely parks throughout the Cincinnati area with most of them having picnic shelters of some kind. Some of the parks have shelters that can be reserved ahead of time and others are on a first come, first served basis, so definitely plan wisely.