STO sells a lot of oil and gasoline to EU members who pay for it in Euros. Thus the more value that the Euro has, the more valuable it is for STO to be paid in Euros. This is not unlike Saudi Arabia, which gets to buy more things with a stronger US Dollar.
With the Euro under pressure, the end result should be that the price of a barrel of Brent Sea Crude should go higher. It isn't. I believe that the markets are not trading on this fundamental right now (otherwise STO would be going up - not down). Thus when the Euro recovers or at least stabilizes, the relative value of the Euros it receives will be greater and thus the price of STO stock should bounce back as well. I believe that the market is trading on this fundamental right now.