Some people have asked me why Delegation is not an option. Unfortunately, there's a limitation that was found in SharePoint Online, where there doesn't seem to be permissions on tasks that are generated by a workflow. This pretty much removes the idea of delegation, since anyone is able to respond to a task that is outstanding. But if another user responds to that task, the AssignedTo property does not get updated. What that means, is that if you assign a task to User1 and User2 somehow opens that task form and responds, when you look at the workflow history, since it says it's assigned to User1, it will look like the task was approved/rejected by that user.
So to having something like Delegation in Office 365, you can update the Assign a Task action, so that the email that goes out, lets the Assignee know that to delegate the task, they can simply forward the email to another user. eg. User1 forward the email to User2.
When User2 responds to the task, the workflow will then to query that task, compare the AssignedTo field to the ModifiedBy field. If they are different, update the task AssignedTo value with the ModifiedBy value. That way, when you look at the workflow history, it will now say that the task is completed, and it will say the correct user associated with that task.
You can see in the workflow history below, that in the History message, the original task was assigned to VadimT, but RichardR responded. The workflow the updated the AssignedTo property, which you can see in the Tasks information.
Here's what the workflow looks like. It's not that hard to add this into a workflow. You can also remove the Log actions to reduce the size of it.