Can I use mainly oyster shell plus little coco coir ?
Better to use just coco coir. The coir holds water which will help to raise humidity.
@hermannicheis may knowEverything I've read says to keep them dry, no humidity. According to Dr. Lafeber: The Egyptian tortoise is native to an arid climate and cannot tolerate damp conditions. Strive for an ambient relative humidity between 20% and 30%. A shallow water bowl should be offered and changed regularly.