This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Yes, annotating a method with the @OneWay annotation is enough if you do Java-first development.
If you are doing WSDL-first development, then (assuming WSDL 1.1) you must not have <output> element(s) or <fault> elements in the <operation> element in question in the <portType> element. The example below has two operations, only the second is a one-way operation.
Oneway operations are useful, for instance, if a client wants to send a message to a web service but is not interested in waiting for a response, for instance with asynchronous operations that may take a long time to finish. The response may later be delivered using callback or the client may poll for the result.