Drut,
The new injector harness comes with o ring already on it, but new ones can be purchased from an decent automotive supplier if required on a clean up of the old one to be kept as a spare.
On the oil pump bolt do a quick search for related topic. Was an issue with some 99-2002 TD5s coming out of the factory with the bolt not loctited in. Is usually a matter of dropping the sump, pulling the bolt out, checking for loctite residue and resecurring with loctite. About $200 for the dealer to do inc new gasket. About $50 to do at home.
Have heard of about 6 that have had loose bolts and 3 that have done engines because of the bolt coming out while driving. Mine was tight.

