Swing (9)
'swing' is the definition.
(oscillating is a kind of swinging)
This is the entire clue.
(Other definitions for oscillate that I've seen before include "sway" , "Swing between limits" , "Rock" , "Move to and fro like a pendulum" , "shake" .)
(Other definitions for era that I've seen before include "Long historical period" , "US equality law" , "Are involved in time span" , "Major division of geological time" , "Long period of history" .)