While cropredy's answer cites the correct documentation:
The style class used to display the output field component, used primarily to designate which CSS styles are applied when using an external CSS stylesheet. This attribute may not work for all values. If your text requires a class name, use a wrapping span tag.
From this VF documentation on styleClass property https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_outputField.htm
His solution is invalid. The documentation refers to the use of a wrapping span tag, which should wrap the apex:outputField tag itself. You must still provide a valid value in the "value" property.
The correct implementation is:
However, the use of a span tag here is probably limiting in most cases. The use of an outputPanel with layout="block" provides a div tag which can be more helpful.
As well as placing the apex:outputField in a pageBlockSectionItem grouping, as the other poster noted, should that fit the situation.