Works by: Drag-and-drop, right-click
Hotkeys: Command+C, Command+P
Source requirements: One or more source relationships required
Target requirement: One target required
New relationships can be added by drag-and-drop, or via the right-click menu.
To create one or more new relationships, select a collection of source relationships that you would like to relate to a new term. Then, select a target term to which they will be related by right-clicking the target term or by drag-and-drop.
From the right-click menu or the drag and drop popup menu, you may add new relationships using the "Add parent term" or "Add child term" menu options. Note that the names of these operations refer to changes that will occur to the target term. So if you drag term A onto term B and choose "Add parent term", A will become a new parent of B.
Both the "Add parent term" and "Add child term" menu items have a flyout submenu of the relationship type that should be used when creating the new relationship:

Select the relationship type you would like for the new relationship, and it will be created. If you would like the new relationship to use the same relationship type as the original source relationship, use the "Use original type" menu option. This is particularly useful if you are creating several relationships at once by using a number of source relationships. If "Use original type" is used with untyped source relationships (ie one or more of your sources is an ontology root) the is_a relationship is used.
When dragging, you can use hotkeys to specify the "Add parent term" or "Add child term" edit operations. To specify "Add child term", press the "Command" and "C" keys on the keyboard, then start dragging. The hotkey dropdown menu will change to "Add child term (copy)" and the title above the terms you are dragging will change from "Dragging" to "Add child...":

When the term is dropped, the type selection menu will pop up.
To specify "Add parent term", press the "Command" and "P" keys on the keyboard, then start dragging. The hotkey dropdown menu will change to "Add parent term" and title above the terms you are dragging will change from "Dragging" to "Add parent..."
Note that if you want to use hotkeys to drag from one Ontology Editor Panel to another, you must specify the "Add child" (or "Add parent") operation in both Ontology Editor Panels.
You can also use hotkeys to choose which relationship type will be assigned when you drop the dragged terms. Each relationship type is assigned a hotkey (Command+another key, usually just the number that corresponds to the relationship type's position in the list, but IS_A always gets the hotkey "I").
Adding a new relationship type is a special case of adding a new term.