What you have might not be an oto. Try looking into chinese algae eaters
or siamese algae eaters
. There are a lot of fish with that general look. We need to figure out what your fish is first before we can tell you what else you can add to the tank.
There are not many fish suited for a 5 gallon tank. Betta is the easiest one. Dwarf puffers and some species of micro rasbora are suitable but dwarf puffers have specific dietary requirements and micro rasbora are sensitive fish. Betta are your best bet. Try to get a healthier fish next time. And see to it that your tank is cycled before adding any more fish.