Parkin scored in three of Fleetwood's five games as they went unbeaten in January to close in on the top of the table.
Oxford keeper Clarke didn't let in a single goal in the Us' four games as they maintained their title challenge.
Rovers may be struggling towards the bottom of the table, but O'Toole found the net three times from midfield last month.
Two of those three came in a match-winning brace against Exeter City that included a 90th-minute winner on the volley.
Finally, Roberts scored in four of Chesterfield's five games as they soared to the top of the table.
The winner will be announced on Friday.