You have a couple options. You normally plan to have 1.5 times the deco gas you need so that you have a healthy reserve. In that regard an AL 80 of EAN 80 is overkill so, if near full, you'd have a lot in reserve.
(An AL 80 of EAN 80 is a lot as using most of it on a single dive, would mean a LOT of deco and would put you over the single exposure O2 limit on a dive with a back gas with a PP02 of 1.2 to 1.4 on the bottom. Divers using an AL 80 with a high precntage deoc gas usually do so with the idea of using the same deco bottle over a few dives on a single trip.)
Option one would be to just roll the valve on and off for each breath to minimize the leakage as with a healthy reserve you could porbably offset the gas lost through the leak.
Option two would be to just go to your lost gas contingency deco plan. If diving on a single deco gas schedule, you ensure you have enough reserve backgas gas to do the longer deco required on the back gas. If you are on a dive with two deco gasses, you would have a deco schedule for each of them independent of the other. Either way, you always have a plan to complete the deco in the event you lose a deco gas. It works pretty well as when the deco gets too long to comfortably do it off the back gas reserve (normally a third of the total backgas) the dive is usually deep enough or long enough where a second deco gas makes sense in terms of shortening the deco obligation.
A third option is team dependent but would involve borrowing your buddies deco gas. That would put you both on a hybrid schedule, in this case, of splitting the deco on back gas and EAN 80. It's not my approach but some divers plan it that way.
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As an aside, I am not 100% clear on what o-ring failed - the DIN o-ring on the face fo the connection, or the o-ring inisde the regulator body that seals the DIN retainer to the reg body?
In the case of the former, the only failures I have seen occur ocurred when the reg depressurized during the dive and the fitting spun itself loose a turn or so - but it can normally be corrected by just tightening the fitting again as the o-ring does not normally get dislodged.
In the latter, the failure would be exceptionally rare as it is a static o-ring. If a large leak like that occurred, I would suspect a loose DIN retainer that created the gap required for the o-ring to extrude and fail.
