TUDIA Ultra Slim Case
Tudia is another brand I've used a lot because they have simple yet extremely thin cases in a nice matte finish for most phones. The TUDIA Ultra Slim case for the OP3 or 3T is a simple TPU case that offers more protection than most at a lower price point. There isn't much to say, other than get one of these if you want a plain yet very effective case for the phone.
Buy it Now for $9.90 in four colors