The 26-year-old prop, who has 17 Ireland caps, was sent off for a dangerous clearout on Munster centre Alex Nankivell during his side's 22-19 defeat at Kingspan Stadium last month.
The United Rugby Championship issued a statement on Monday saying O'Toole had breached a law relating to dangerous play in a ruck or maul - "a player must not drop their weight onto an opponent or target the lower limbs".
O'Toole, who featured for Ireland in the November internationals, will be suspended for the defending champions' Six Nations opener at home to England on February 1st and the away game against Scotland on February 9th.
He will also miss three further Ulster games, having already sat out the trip to Connacht on December 28.
O'Toole escaped a 10-game ban due to his "admission of foul play, immediate apology and again post-match", the URC said.
The statement added that Ireland interim head coach Simon Easterby had confirmed O'Toole had a "very good chance" of being named in matchday squads for the Six Nations.
"In these circumstances it is appropriate that Ireland's first two matches are included," it said.