James MacDonald, in Essex, England, digs deeper into the Steinbeck-O'Hara connection:
The Steinbeck-O'Hara connection is intriguing for the reasons you suggest.
But then the pre-war John O'Hara was pretty much a New Dealer. In 1964, anyway, he wrote James Gould Cozzens that he and Steinbeck had been friends since 1936, when he was seriously considering adapting In Dubious Battle for the stage. Steinbeck wrote to O'Hara in 1951 in defence of The Farmers Hotel (which had been given a critical roasting); then, in 1957, Steinbeck was one of O'Hara's principal sponsors for his election to the National Institute of Arts and Letters.
O'Hara's telegram to Steinbeck after Steinbeck won the Nobel Prize was: "Congratulations. You were my second choice." And O'Hara drove to Sag Harbour, on Long Island, New York, to read to Steinbeck after eye surgery. Steinbeck's widow attended O'Hara's funeral, and there is a well-known photo of O'Hara and Steinbeck at Quogue, though Steinbeck's appearance looks decidedly more plebeian than O'Hara's.
What impresses me most about the friendship, such as it was apparently, is how candid they must have been about each other's work. O'Hara was very critical of the supposed verisimilitude of East of Eden, for example, I've long wondered how they could have sustained friendly visits with that level of disagreement between them. It certainly speaks well of Steinbeck's temperament.
Matthew Bruccoli says that O'Hara tended to look on his contemporaries as members of a club, and this is how he viewed John Steinbeck, whatever he may have thought of his individual novels. It is always gratifying to me how much fellow novelists respected O'Hara.
I share your misgivings about the Library of America's continuing disregard of O'Hara. I am a LOA customer and have written them about O'Hara. They've included far inferior authors, God knows.