Ann: DLS: Bidder's Statement, page-6

Drillseach cannot make on market purchases for two reasons. Drillsearch has already reach the 20% threshold more or less. Drillsearch cannot pay more than the offer price.
90% is the compulsory acquisition threshold.
Magnum can buy on market provided it is not above the implied offer price that they have stated. I doubt that many AQO big holders would be keen on Magnum Hunter shares. This give Drillsearch a window to pounce quickly by doing a deal with the big holders of AQO.