The title means a miscellaneous collection, and that’s what playwrights Theresa Rebeck and Alexandra Gersten-Vassilaros have crafted. By turns a brittle comedy, existential drama, and paean to haute cuisine, ”Omnium” gathers assorted stereotypes — a conservative Tom Clancyish writer, a Martha Stewart-like hostess, et cetera — at a post-9/11 dinner party where they, inevitably, clash and burn. (Think Sartre’s ”No Exit,” but with better food.) There’s a lot of holier-than-thou ranting — Israel, Iraq, even The New York Times gets taken apart. But it’s the rarer, quiet moments that ring true: two men sharing a toast; an improbable ode to creme brulee; a dim-witted fireman recalling with disarming eloquence the day ”death came down on all of us.” After that, the histrionics seem superfluous.