My betta obviously ate the half of the algae disc I threw in for my Oto....

His stomach is extremely big now so I'm going to fast him for a bit, will he be ok?

Now I'm concerned for my Oto, if the Betta gets the disc then what about her? What can I do since the Betta flares at the Oto and so the Oto must be scared to eat her own food.

 

I'd remove the Oto if the Betta is flaring at it, it might decide to do more than just flare & hurt the Oto.
Oto's are also a shoaling fish & should really be kept in groups of at least 6 & you haven't room in a 10 gallon

 

Also, keep in mind bettas are not community tank and are solitary and really should be the sole specimen in their own tank.

 

Yes, if the betta is harassing the oto I'd remove her. Sometimes it works, sometimes it doesn't.

 

Agreed, I'd move the oto to a bigger tank and get at least 6 of them.