O-             Starter of many an Irish surname

OT-            NFL period if game is tied in regulation

ROT-            Decay

TRIO-            The Stooges, The Musketeers, or the Little Pigs

RATIO-        π vis-a-vis circumference and diameter

AORTIC-        Related to the largest artery

EROTICA-        Titillating literature

REACTION-        Per Newton, an equal and opposite event

CREMATION-        Alternative to a casket burial

IMPORTANCE-    Being Earnest, in an Oscar Wilde play