org-indirect-buffer-display

org-indirect-buffer-display is a variable defined in `org.el'.
Its value is

other-window

Documentation:
How should indirect tree buffers be displayed?
This applies to indirect buffers created with the commands
M-x org-tree-to-indirect-buffer and M-x org-agenda-tree-to-indirect-buffer.
Valid values are:
current-window Display in the current window
other-window Just display in another window.
dedicated-frame Create one new frame, and re-use it each time.
new-frame Make a new frame each time. Note that in this case
previously-made indirect buffers are kept, and you need to
kill these buffers yourself.

You can customize this variable.