Position oven racks in the middle and top of the oven and heat the oven to 350°F. Line two large baking sheets with parchment.
In a large bowl, whisk the flour with the sugar, baking powder, and salt to combine. Put a bit of the flour mixture in a small bowl, add the orange zest, and rub the zest into the flour to keep it from clumping. Stir the coated zest and the pecans into the rest of the flour mixture.
In a small bowl, whisk the eggs with the olive oil, orange juice, and liqueur until well blended. Pour into the center of the flour mixture. Stir with a wooden spoon until the dough is blended. The dough will be very sticky.
Dump the dough onto a heavily floured work surface and divide into six equal portions. Roll each portion into a log that’s 12 inches long, dusting with flour along the way to keep the dough from sticking. Set the logs about 3 inches apart on the prepared baking sheets and then press gently to flatten each log so that it’s 1-1/2 to 2 inches wide.
Bake until the logs are golden and the tops are fairly firm near the center, 22 to 25 minutes, rotating the sheets and switching their positions after 10 minutes to ensure even baking. Set the sheets on racks until the logs are cool enough to handle, 20 to 30 minutes. Leave the oven set to 350°F.
Transfer the logs to a cutting board and, using a serrated knife, saw them on a sharp diagonal into slices 1/2 inch thick. Arrange the slices on the baking sheets, laying them flat with a cut side down. Return the baking sheets to the oven and bake the biscotti for about 6 minutes. Turn the biscotti over, rotate the baking sheets and switch their positions, and bake until the biscotti are golden, another 8 to 10 minutes.
Let cool on the sheets on racks for 5 minutes before transferring them to racks to cool completely (the biscotti will get crisp as they cool).
Store at room temperature or freeze in an airtight container, separating the cookie layers with waxed paper.