The cast is spot-on. Is there any ingenue role in musical theater that Kelli O’Hara — of “The Pajama Game” and “South Pacific” revivals — couldn’t make her very own? In “Nice Work,” even given a stellar cast, when she’s on the stage she often is the single focus, by sheer force of her ability to sing any song fully in character, and deliver it with a striking musicality. This leaves her leading man, Matthew Broderick, in an uncomfortable position. Although his part of a rich playboy with a low-wattage brain means he must appear as a constant shade of gray among the colorful characters on stage, a part he delivers earnestly, his singing seems only serviceable by comparison to O’Hara’s.