In the bowl of a stand mixer fitted with a dough hook, combine water, yeast, flour, sugar, salt, 1 large egg, and oil.
Mix until dough comes together and forms a smooth and elastic ball (about 5 minutes). Add more water if the dough is too dry. If the dough is too wet, add a little more flour.
Pour the dough into a large greased bowl and cover it with plastic wrap that has been sprayed with cooking spray. Let it rise in a warm place for about 1 ½ hours, or until doubled in size.
Line 2 baking sheets with parchment paper.
After the dough has doubled in size, punch it down to remove excess air.
Divide the dough in half. Then, divide each half into 3 equal portions for a total of six portions.
On a lightly floured work surface, roll each section into a long rope, about 16 inches long.
To braid the dough, select three ropes of dough and place them vertically side by side. Then, pinch the three ropes together at the top. Bring one strand from the left and place it over the strand in the center. Then, bring the strand from the right and place it over the strand that is now in the center. Continue this process until you have an even braid. Pinch the last part of the braid together. Repeat with the other three ropes of dough.
Place each of the braided challah loaves onto a lined baking sheet. Cover each with plastic wrap that has been sprayed with cooking spray. Let them rise for another 30 minutes, or until doubled.
When ready, preheat your oven to 350°F.
Add the remaining egg to a small bowl. Add 1 tablespoon of water and mix until blended. Use a pastry brush to brush the egg wash evenly over the tops of the loaves of bread.
Bake for 25-30 minutes, or until the bread is golden brown and reaches an internal temperature of 190°F on an instant-read thermometer.
Allow the challah to cool for 15-20 minutes before slicing and serving.