Given Thorondor's sibling comment to yours:
Alpha Centauri A has a mass of 1.1x the sun, and is about 4.37ly away, which would mean our sun's gravity well runs out at
(1/2.1)*4.37 = 2.08ly - i.e. any object in orbit 2.08 ly away would (even if affected by nothing else), fall into Alpha Centauri A's gravity well before completing a full orbit and therefore couldn't possibly remain in orbit around our sun.
If that's true (and I have no idea whether any of it is true), then I think 2.08ly must be an upper bound on the outer boundary of the Oort cloud.
EDIT: Ah, I'm thinking 2-dimensionally. Of course, something could be in a much larger orbit in a plane that doesn't go near Alpha Centauri.