Sprinkle katakuriko potato starch and salt over the oysters. Gently rub them to clean. Wash them with water and strain, then pat them dry with paper towels to remove excess moisture.
In a large bowl, whisk together flour and dashi broth until smooth. Stir in the egg and grated yam until well combined with the flour mixture. Season the batter with salt and soy sauce. Reserve 1/2 cup of the batter. Thinly slice cabbage, and add it to the rest of the batter in the large bowl along with the tenkasu tempura bits. Mix everything together until well combined.
Heat oil in a skillet. Pour half of the pancake batter with cabbage into an 8-inch circle in the skillet, and cook until slightly set. Arrange half of the oysters in the skillet to one side of the pancake and cook until browned. Place the cooked oysters on top of the pancake, then pour half of the reserved batter (without cabbage) over the oysters. Continue cooking over medium heat until the bottom of the pancake becomes golden brown, about 7-8 minutes. Carefully flip the pancake over and cook for another 7-8 minutes until the other side is golden brown and the pancake is cooked through.
Flip the pancake one more time so that the oyster side is facing up. Spread Okonomiyaki sauce over the pancake, then top it with chopped green onions and pickled ginger. Repeat the process to make another pancake.