OpenBSD's Linux/FreeBSD binary emulation layer is only for i386, and, it's mostly unmaintained, given the recent events at the c2k10 hackathon, if nobody steps up they'll probably be removed entirely in a few releases.
You can't run Opera on OpenBSD/amd64, not unless they release a native version.
There was originally a focus on binary emulation early on in the project, but as of late, this has been rethought.. as Theo de Raadt said, this is a source code world.
http://marc.info/?l=openbsd-cvs&m=127836896111695&w=2