Note: Since making this video I have found the best O-Ring replacement is "4mm (ID) x 2mm (CS) silicone o-ring" I have seen packs of 10 for under £3 online.
(Update: Dec 2022)
It may be worth considering changing the whole "DeLonghi Infusion Piston Plate", some traders call it a "Coffee Maker Generator", as there are many other o-rings that can wear out, and the infuser piston and pipes get grotty over time. It will make your coffee machine like new and only costs a little over £30 (UK). It will be a bit more effort, I'm told the pipes are hard to refit, but I think well worth the effort. Obviously check that it fits your model as there are a few types.
This was a guide for my Uncle, But it might be useful for a lot of
Delonghi "bean to cup" machines that are dumping the water into the drip tray via the overflow pipe instead of through the infuser.
Machine being repaired:
Delonghi Magnifica S
Model: Ecam 223.320.sb