The scene in which Bigelow runs in panic through the streets after learning he has been poisoned was a stolen shot. The pedestrians had no idea a movie was being made and no warning that Edmond O'Brien would be plowing through them.
The Phillips Export-Import Co. where Bigelow is sent in Los Angeles has its offices in the Bradbury Building, an iconic office building in Los Angeles designated a National Historic Landmark in 1977 and the scene of numerous movie shoots from the 1940s into the 2000s. Among the most famous movies with scenes shot in the Bradbury are Blade Runner, Chinatown and I, the Jury.
When Frank Bigelow registers at the Allison Hotel in Los Angeles, the name directly above is Russell Rouse, one of the writers. Also on the register is Ernest Laszlo, the director of photography and Marty Moss, the assistant director.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
This is Neville Brand's first credited part, not--as is often claimed--his debut. He had small, uncredited roles in a few films before this one, such as Port of New York (1949) and Halls of Montezuma (1951) (which was made before this film but released after it).
Pamela Britton plays Paula Gibson, the insecure and pushy girlfriend of Frank Bigelow, played by Edmond O'Brien. In 1951 Britton would be cast as Marge Porter in TV's The Bigelow Theatre (1950), a series of teleplays, but which had nothing to do with the name of Edmond O'Brien's character in this film.
Just for fun: Back in the 1980's, there was a short-run independent TV series that provided voice-overs with new dialog and plots for old movies. For D.O.A., Edmond O'Brien was "Rickie," Pamela Britton was "Ethel," and "Rickie" was running around California trying to find "Lucy," who was kidnapped or something. When "Rickie" finally collapsed at the end at the police station, the file was stamped D.O.A. in bold letters, with the title on the screen stating "D.O.A.: Death from Over Acting."