.OP is not a GNU extension but first appeared in the Documenter's Workbench,

see https://github.com/n-t-roff/DWB3.3/blob/master/macros/man/an.sr#L33

from g dot branden dot robinson at gmail dot com
via GNU troff commit e3459327 2 Aug 2021 10:49:46 -0400

diff --git a/share/man/man7/man.7 b/share/man/man7/man.7
index 5eb4e40..6eb25e4 100644 (file)
--- a/share/man/man7/man.7
+++ b/share/man/man7/man.7
@@ -1,4 +1,4 @@
-.\"    $OpenBSD: man.7,v 1.59 2020/10/28 15:31:32 schwarze Exp $
+.\"    $OpenBSD: man.7,v 1.60 2021/08/05 14:31:06 schwarze Exp $
 .\"
 .\" Copyright (c) 2009, 2010, 2011, 2012 Kristaps Dzonsons <kristaps@bsd.lv>
 .\" Copyright (c) 2011-2015, 2017-2020 Ingo Schwarze <schwarze@openbsd.org>
@@ -17,7 +17,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: October 28 2020 $
+.Dd $Mdocdate: August 5 2021 $
 .Dt MAN 7
 .Os
 .Sh NAME
@@ -246,7 +246,7 @@ link description to be shown
 .Ed
 .It Ic OP
 Optional command-line argument.
-This is a non-standard GNU extension.
+This is a non-standard DWB extension.
 It has the following syntax:
 .Pp
 .D1 Pf . Ic OP Ar key Op Ar value
@@ -516,7 +516,7 @@ The syntax is as follows:
 .It Ic I   Ta    n         Ta    next-line Ta    \&
 .It Ic IB  Ta    n         Ta    current   Ta    \&
 .It Ic IR  Ta    n         Ta    current   Ta    \&
-.It Ic OP  Ta    >=1       Ta    current   Ta    GNU
+.It Ic OP  Ta    >=1       Ta    current   Ta    DWB
 .It Ic PD  Ta    1         Ta    current   Ta    \&
 .It Ic RB  Ta    n         Ta    current   Ta    \&
 .It Ic RI  Ta    n         Ta    current   Ta    \&