The hedge fund bought 7mln shares for 5.25p and now owns 1.51% of the cash-strapped fertiliser company.
It is the third time in a week the hedge fund has bought shares in the York Potash project owner, with all of its purchases so far at prices below the takeover offer from Anglo American.
The FTSE 100-listed miner is looking to snap up Sirius for £405mln, or 5.5p per share.
There is a shareholder meeting on 3 March (next Tuesday) to vote on the bid.
While the board has recommended investors approve it, half of Sirius shareholders are private investors who bought shares at a much higher price and claim the offer “significantly” undervalues the company.
Odey has also openly criticised Anglo's offer at the current price, saying it might fail or be derailed by a third party.
“The lack of ‘final’ offer, in Odey’s opinion, suggests that Anglo American would be willing to bid substantially more for Sirius.
"The investment case remains highly attractive for Anglo American, even at a materially higher bid level,” the fund manager said earlier this month.
Shares in Sirius were trading at 5.22p on Wednesday afternoon.