Some tips for increasing survival rates of newly purchased otos that I find work for me.
(1) Resist the temptation to buy otos that have not been in the LFS tank for at least two months. The ones that are the weakest, ill, and most stressed out will usually kick the bucket in the the first month or so, leaving the hardiest of the bunch.
(2) Choose the fattest otos you can find. A pot bellied oto(within reason, you don't want one with dropsy), means that it is well fed and eating, so more likely to survive.
(3) Never put these guys in an uncyled tank, this should be common knowlege when popuating tanks with most fish. Otos are especially sensitive to ammonia and nitrite spikes.
(4) Drip acclimitize them for at least an hour, preferrably two hours. Yeah, I know this sounds drastic and extreme, but IME, it is one of the most effective ways of increasing survival rates for otocats, and pretty much any other fish.