Otos are found in South America living in the Amazon river. So it isn't hard to create a tank environment that they will like. If you want them to truly thrive and be happiest however, you should keep them between 75F and 82F. PH between 5 and 7.8 (though if you want them to breed, keep it below 6.8), with soft acid water.They LOVE currents and fast moving water, currents as this helps keep the oxygen-rich water they prefer. They also seem to like hanging out directly under the filter where the current is strongest for part of the day, when first introduced to your tank.
Be sure to include lots of hiding places, as otos are easily startled. The females especially I have found will dart away at the slightest hint of movement towards them. They also LOVE live plants. While you can have them in a tank without plants, they will be much happier with them. And of course if you wish to breed them, you lose almost all chance of this happening unless you have plants.
As well, if you house otos in non-planted tanks, you will need to feed them suppliments more often as the tank will not have the algae growth needed to keep them healthy.