This was an idea I put in the BIONICLE 2015 Discussion topic, but I was told it was had no place there. But now that you've created this topic, it may have a place.
(NOTE: If you have seen this idea before, I've edited it a bit).
Maybe the power of Mata Nui's robot and the Mask of Life not only restored life on Spherus Magna... but also created Okoto.
Now here's my reasoning.
See the ocean behind Tahu in this picture?
I thought, "If there are still large oceans on Spherus Magna, is it possible there could be places even the G1 Toa don't know about? Things created by the Mask of Life and Mata Nui in their last move to restore life to the planet? An island? Perhaps... Okoto?"
Or maybe Okoto (or the pieces which would come together to form Okoto,) broke off from the isle of Mata Nui (like Voya Nui),
and two brothers the Mask Makers founded Okoto? Using the Mask of Control, maybe Makuta managed to control the island's broken fragments to bring them together. Then, Ekimu used the Mask of Creation and created a series of mountain ranges to hold the island (Okoto) together.