i don't even know that the dealer has a part number for this, but you can look it up on Subaru's opposed forces website or call. they probably just use a rebuild kit or replace the power steering pump unfortunately.
This oring size will work if you want to order one from online or a local store:
1/2'' ID, 5/8" OD, 1/16" Thick
the dealer could look it up for you on their system, but many people have used hardware store or car parts store o-rings for that.
at present I have one of those turquoise-colored ones in there from an assortment pack sold for A/C system repairs.
Maybe as well inspect the hose and consider replacing those two spring clamps with screw-type hose clamps. protect the accessory belt from spillage with a rag. Don't over-torque the bolt when replacing the fitting.
Consider swapping out a tankful of fluid at least. ATF Dex III