Over the past few years I have been developing custom workflow actions - for use in SharePoint Designer workflows - for SharePoint 2007, 2010 and 2013 (on-premise). See this example for the kind of workflow action I am talking about.
I occasionally get questions from people who want to use similar actions in SharePoint (2013) online. I am aware that in the 2010 version of SharePoint online there was the possibility to (painfully) create sandboxed workflow actions, but that no longer appears to be an option in the SharePoint 2013 version.
I fear that the answer is 'it is not possible', but is there any way to create and deploy a custom workflow action to SharePoint online for use in SharePoint Designer workflows? I cannot find any documentation on the topic.
Update 1: Apologies for not making this clear, but I need it to work with SharePoint 2013 workflows (not workflows in SharePoint 2010 mode) and deploy it as an App to SharePoint online (so no sandboxed solutions)
Update 2: I have opened a support case with Microsoft (113072910624379) and it is official, SharePoint Designer Workflow Actions cannot be deployed as part of an App. A sandboxed solution using declarative Workflow Actions is the only way to do this.