Suncor Energy has extended its C$4.3bn ($3.2bn) takeover offer for its Canadian Oil Sands (COS) until 8 January 2016.
The offer was originally set to expire on 4 December 2015.
The oil market outlook has worsened since the company first expressed interest to acquire Canadian Oil Sands.
"The pressure is clearly on COS' board and management to prove they are acting in someone's interest other than their own."
Canadian Oil Sands also reported negative free cash flow for the first nine months of 2015 and announced an optimistic production outlook for 2016 recently.
Suncor Energy president and CEO Steve Williams said: "The pressure is clearly on COS' board and management to prove they are acting in someone's interest other than their own.
"The Alberta Securities Commission decision allows COS more time to surface a superior offer from a credible third party, something most analysts see as unlikely.
"Suncor's offer delivers a significant and immediate premium, continued exposure to rising oil prices, and superior long-term upside versus the significant risk facing COS and its shareholders."
The company urged shareholders of COS to tender their shares to the offer and act to protect the value of its investment.
Suncor said that all regulatory conditions to the offer, including those related to the competition act (Canada), have been satisfied.
In October 2015, Suncor Energy made an unsolicited offer to acquire all of the outstanding shares of COS.
The deal value is expected to rise up to $6.6bn ($5bn), as Suncor Energy has offered to assume $2.3bn of COS' estimated outstanding net debt as of 30 June 2015.