Each of the Project's utilities may be slightly different, as each has a different provenance.
OpenBSD's dd(1) is based on NetBSD's, as OpenBSD was itself a fork of NetBSD, a little over 20 years ago. It has been continuously maintained, with the last revisions in support of OpenBSD's new pledge(2) security framework.
Here's a link to the source code:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/bin/dd/