Sorry to hear about your oto. They're very nice little fish, but a lot of people have trouble keeping them.
Ideal temperature for a betta is going to be 76-82 Fahrenheit, whereas neon tetras do better between 74 and 80.
Both neons and otos do better and are more active in larger numbers. You might want to consider getting 3 or 4 more neon tetras. If you do, you should quarantine them for at least 2 weeks before putting them in your tank, since neons seem to be very susceptible to disease.
EDIT: My internet connection was screwy. sorry about that. Your temperature should be fine where it is.
Last edited by Raaan; 12-13-2014 at 09:49 PM.
Reason: Correcting doubled text