Orison S. Marden, the lawyer whose career you correctly describe in
your question was the son of Orison Swett Marden, the founder of
Success Magazine and author of motivational books.
I obtained the information from Marden's obituary in the August 26,
1975 N.Y. Times. For copyright reasons, I can't reproduce that
obituary here, but it is available for purchase for $2.95 at the New
York Times website. It also might be available to you through a local
public or university library. To access it at the New York Times
website, use the search box at the top of the Times's home page,
New York Times
Then, choose "NYT 1851-1995" and search for "orison s. marden."
Here is the brief reference in that obituary to Marden's father:
"Orison Swett Marden was born in Sea Cliff, L.I. on May 22, 1906. His
father, after whom he was named, was a physician as well as a lawyer
and a writer of books on how to achieve success in life. His mother
was the former Clare Evans."
You are certainly right that this information seems unusually
difficult to find online without determining Marden's date of death
and looking in subscription-based newspaper archives. After failing
to find the information with various Google searches, I determined
Marden's date of death by searching at the Social Security Death
Index, which is at this linked site:
Social Security Death Index
Since his name is so unusual, a search at this site turned up only one
"hit," which indicated that Marden died on August 25, 1975. I then
accessed the New York Times archive through my local public library
and searched on Marden's name for a period including that date.
I am confident that this is the correct answer to your very specific
question. If anything is unclear, please ask for clarification before
rating the answer.