I wouldnt add any tetras for two reasons. you say its "just getting going" so I assume its a new setup. Firstly. is the tank cycled? If not I wouldnt add any more fish til you know the tank is cycled...please check out the beginners section
http

/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/
also,if it is cycled, tetras dont tend to do well in new tanks. they need the tank to have been setup for 6 months to be able to be comfy
also, bettas are aggressive, thats why they are called fighter fish. anything with long flowy, colourful fins and striped bodies will be viewed as another betta, and the betta will slowly kill each one of them. Especially if they are around the size of a female betta...he will squeeze them to death trying to mate with them!
i had pygmy corys in with mine, and as they are a fast fish compared to bettas who are much slower, they can out swim them. but make sure all inlets to any filter are covered with a stocking end ( pair of tights cut off at heal) and use elastic bands to keep them in place ( these will need replacing as rubber perishes in water)
Another idea for a tank mate for a betta, is an apple snail. they add colour to the tank, and are too hard for betta to attack and injure. I also managed to keep any corydoras species with betta, a minimum of 4 would be great, but take into account they can grow to 3 inches each when adding up your inches for stocking control reasons. ( 1 inch of adult body length, not including fins, to each gallon of tank water)
sp at the moment you have a male betta (2 and a half inches) with two oto, which are two inches each full grown...so you already have 6 and a half inch of stock, leaving you with 3 and a half inches of stock allowed. I would suggest another oto, but feed algae flakes regularly, or another algae eater of some sort. or 4 pygmy corys than dont quite reach an inch in length at adult size, so you may get away with that#
good luck...any pics of your betta? I love them xx