org-startup-folded

org-startup-folded is a variable defined in `org.el'.
Its value is

t

Documentation:
Non-nil means entering Org-mode will switch to OVERVIEW.
This can also be configured on a per-file basis by adding one of
the following lines anywhere in the buffer:

#+STARTUP: fold (or `overview', this is equivalent)
#+STARTUP: nofold (or `showall', this is equivalent)
#+STARTUP: content
#+STARTUP: showeverything

By default, this option is ignored when Org opens agenda files
for the first time. If you want the agenda to honor the startup
option, set `org-agenda-inhibit-startup' to nil.

You can customize this variable.