This may be too old of a post… but, it seems as though what you are looking for is:
root php include (or same directory include):
include(file.php);
osc_includes/ php include (or higher directory include):
include(../osc_includes/file.php);
“../” will take you back one level in the directory tree. You can combine these to, such as “../../file.php” if you must go up two directories higher.
