The Broadway transfer of J.T. Rogers' Oslo opens April 13 at the Vivian Beaumont Theater. Helmed by Tony winner Bartlett Sher and headlined by Tony winners Jennifer Ehle and Jefferson Mays, the production began previews on March 23. A darkly comic epic, Oslo tells the true-but-until-now-untold story of how one young couple, Norwegian diplomat Mona Juul (Ehle) and her husband social scientist Terje Rø…