Na FAQ (na verdade):
13.14 - Can I have Flash support in my web browser? (i386 only)
Firstly, if you are just looking to watch flash videos from common websites, there are a number of options in packages, including: get_flash_videos, minitube, youtube-dl, get_iplayer and yt.
The Flash plugin is distributed by Adobe in binary form only. Adobe does not provide a native OpenBSD plugin, but there is a Linux plugin which you can use under Linux emulation. This plugin is available only for the i386 platform.
Before continuing, it is a good idea to read about Linux emulation in the compat_linux(8) manual page, and also FAQ 9 - Running Linux binaries on OpenBSD.