Hi!
This is quite easy. First you have to create a field, for example a choice field for keeping the outcomes. So it will have at least two values from which approver will be able to choose: Approve, Reject.
Then you need to use wait action: Wait for Event in List Item . Your approver then visits the item's display form, then goes to edit form and in that form chooses appropriate option from you choice field. Before he does it your workflow is waiting. After - the wait action is executed and then you need to query your item to get the outcome. And then, based on the outcom you use the action: condtional branch in office 365 and develops your further logic 
Regards,
Tomasz