Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Finely chop the fresh spinach and mince the garlic clove.
Sauté the spinach and garlic in a non-stick pan over medium heat until the spinach is fully wilted, then transfer to a fine-mesh sieve and press out all excess liquid.
In a medium mixing bowl, combine the shredded cooked chicken breast, wilted spinach, crumbled feta cheese, egg, sea salt, black pepper, and dried oregano.
Lay one sheet of phyllo dough on a clean work surface and brush very lightly with half of the olive oil, then place the second sheet directly on top.
Cut the layered phyllo sheets lengthwise into three equal strips.
Place a third of the chicken and spinach mixture at the bottom end of each strip.
Fold the corner of the phyllo over the filling to form a triangle, then continue folding like a flag until the entire strip is used.
Place the triangles on the prepared baking sheet, brush the tops with the remaining olive oil, and bake for 18-20 minutes until the pastry is crisp and golden brown.