In my package, i’m trying to put everything under /opt/%{name}/*, but OBS tells me that this is not allowed in FHS 2.2. If i understand http://www.pathname.com/fhs/2.2/fhs-3.12.html correctly, /opt/<package> should be perfectly fine however.
This has nothing to do with obs, but with the policy rules, which are installed by openSUSE distro configs by default.
OBS is intendended to be used to build distro packages (so it is easy to build for SUSE, Fedora and friends at the same time) and not for building a single binary which is installed under all (which are intended to use /opt).