You definitely need rain pants, but plenty of people get by just fine with frog togs and things of that nature that tear easily. Usually the trails are pretty well kept, but that may not always be the case for you.
The thing to under stand about OATC is you will be doing a lot of trail work, and so think about what else you're taking as pants. Do you plan on ever wearing your rain pants while working? That could influence the decision quite a bit. A lot of scouts use their rain pants as their only pants, for things like cons projects, so if that describes you, then you may want the more durable ones.