If you need more time to file your Ohio tax return, you only need to file a federal extension. When you later file your Ohio return, answer Yes to the question asking if you filed a federal extension. Ohio does not have a separate Ohio extension request. It honors the federal extension.
To file a federal extension, click on the Personal menu link at the top of the page and go to "2018 Extension".
Note that an extension of time to ﬁle does not extend the time for payment of tax due. If you owe taxes to Ohio, you'll be charged a late payment penalty unless you pay them by the April due date.