Obligation, duty are comparable when they denote what a person is bound to do or refrain from doing or for the performance or nonperformance of which he is held responsible.
In ordinary usage obligation typically implies immediate constraint and a specific reference.
Duty, on the other hand, often suggests less compulsion from immediate circumstances but a greater impulsion on moral or ethical grounds; thus, a person weighed down by a sense of duty is keenly aware of what in general he ought to do; one has a sense of obligation only in a particular case and for a particular reason.