Starting with the 1930 US Census, York, York Co, PA
ED 111, sheet 9B, lines 52-55:

DIEHL, Oscar N. - head, 52 y/o, married at age 24, born in Pennsylvania, parents born in Pennsylvania, occupation: clerk for steam railroad office
-----, Catherine D. - wife, 47 y/o, married at age 19, born in Maryland, parents born in Maryland
-----, Dean K. - son, 20 y/o, single, born in Pennsylvania
-----, Helen M. - daughter, 16 y/o, born in Pennsylvania

1920 US Census, York, York Co, PA
ED 146, sheet 14 A, lines 17-23:

DIEHL, Oscar N. - head, 42 y/o, botn in Pennsylvania, parents born in Pennsylvania, occupation: clerk for PRR
-----, Catherine B. - wife, 34 y/o, born in Maryland, parents born in Maryland
-----, Pomroy D. - son, 15 y/o, born in Pennyslvania
-----, Paul J. - son, 13 y/o, born in Pennsylvania
-----, Dean K. - son, 9 y/o, born in Pennsylvania
-----, Helen M. - daughter, 5 y/o, born in Pennsylvania

1910 US Census, York, York Co, PA
ED 122, sheet 13A, lines 22-26

DIEHL, Oscar N. - head, 30 y/o, first marriage, married 6 years, born in Pennsylvania, parents born in Pennsylvania, occupation: clerk for Railroad
-----, Catherine - wife, 29 y/o, first marriage, married 6 years, 3 of 3 children living, born in Pennsylvania, parents born in Pennsylvania
-----, Pomroy D. - son, 6 y/o, born in Pennsylvania
-----, Nevin O J - son, 3 y/o, born in Pennsylvania
-----, Dean K. - son, 0/12 or 2/12 y/o, born in Pennsylvania

Without knowing Oscar's parents' names, I can't go back any further in census records with any certainty, but there is an Oacar N. Diehl in the 1900 census, born Sept 1878 living in Franklin Twp, Adams County whose parents were James F. and Arabella E., siblings Frederick, John, Robert, Elizabeth, and Samuel - all born in PA [ED 2, sheets 5A&B].

Searching for a WW1 draft registration for Oscar yielded this:

Oscar Nevin Diehl
York, York Co, PA
DOB: 19 Sept 1878
age: 39
native born
next of kin: Catherine Day Diehl

So, it looks like James and Arabella may well have been his parents.

-MJ