Marinate oyster mushrooms with olive oil and seasoning for 1-4 hours.
Preheat cast iron skillet on medium-high heat.
Add oil and mushrooms. Press with a second cast iron skillet or steak-weight grill press. (Both pans will be very hot, so be extremely careful not to burn yourself or the mushrooms.) Sear for about 4-5 minutes, then turn mushrooms over and press again. Sear for another 4-5 minuets.
Reduce heat to medium and cook for about 5 more minutes, stirring occasionally.
Remove mushrooms from pan and let rest for 2-3 minutes before slicing. They should have a very nice char to them, but soft and cooked through on the inside.