While Creating/Editing a task you can link the task to an EPIC.

1. Navigate to Quick Links and click on “Task” to Create Task.
2. Select the required Epic from the dropdown.
3. Enter the other required task detail and Save.

In an Agile project, one can also link a task to an epic from a Backlog Page

1. Navigate to the Left menu and click on “Backlog” to Create Task.
2. Hover on the Task list
3. Select the required Epic from the dropdown

Know more about Epic:

How to Create Epic in Orangescrum?

How to View Epics in Orangescrum?

How to see Epic Progress in Orangescrum?

User Note for Orangescrum Epic View

Source: Orangescrum blog