A common misspelling of opus magnum (or magnum opus). Notice that in Latin opus (gen. operis) is neuter, which means that the adjective that goes with it should be also neuter: magnum.
"What ?", I can hear you shout, "isn't everything that ends in -us masculine ?"
Well, no. If you don't believe me, think of opus infinitum, opus incertum and even opera omnia. I mean, c'mon ! Have you already seen a masculin plural in -a ? Wouldn't it be *operes omnes ?

This node was written as a service: *magnus opus is a frequent mistake, which means that people will search for it. And if they search, it is better that they find something.