Short answer is yes the drop is somewhat problematic as you are get getting STO shares, but typically you look at the value of the shares you are receiving with a longer term view (although the current SP is very relevant), not simply taking the latest SP. That's why you often see VWAP numbers quoted also. There is quite a lot of noise in the current share prices during a takeover and the acquirer share price will potentially increase once the certainty of the deal increases also.
From here its kind of a PR / investor relations battle. STO is trying to get OSH share price down (through leaking the deal, putting pressure on the chairman / CEO etc) and boost theirs (positive earnings, increase positive trading sentiment). The synergies combining the two are also relevant to how much STO can offer until its dilutive to STO earnings. Santos need to help shareholders better understand this number I think.
