You can see how many OP you have in the upper right of the screen.
To start using your OP, click on the OP number. Now, all your machines and animals (so anything that can use OP) will have a small circle, marked either red or green.
Those marked red and say OFF are currently NOT using OP. Those marked green and say ON are currently using OP.
To switch from not using OP to using OP, or the other way around, just click the circle.
Finally, click the OP number on the bottom left of the screen again, to close the OP tool.