Rename this file to patch-tun_c and put it into the patches directory of the openvpn port.
Afterwards rebuild the port. The patch should be applied automatically.

mkdir /usr/ports/net/openvpn/patches
cp openvpn-patch-tun_c /usr/ports/net/openvpn/patches
cd /usr/ports/net/openvpn/patches
make && sudo make install clean

$OpenBSD$
--- tun.c.orig	2004-02-06 11:41:19.000000000 +0100
+++ tun.c	2004-02-06 11:48:41.000000000 +0100
@@ -508,7 +508,7 @@ do_ifconfig (struct tuntap *tt,
        */
 
       openvpn_snprintf (command_line, sizeof (command_line),
-			IFCONFIG_PATH " %s delete",
+			IFCONFIG_PATH " %s destroy",
 			actual);
       msg (M_INFO, "%s", command_line);
       system_check (command_line, NULL, false);
@@ -524,7 +524,13 @@ do_ifconfig (struct tuntap *tt,
 			  tun_mtu
 			  );
       else
-	no_tap_ifconfig ();
+	openvpn_snprintf (command_line, sizeof (command_line),
+			  IFCONFIG_PATH " %s link0 %s netmask %s mtu %d up",
+			  actual,
+			  ifconfig_local,
+			  ifconfig_remote_netmask,
+			  tun_mtu
+			  );
       msg (M_INFO, "%s", command_line);
       system_check (command_line, "OpenBSD ifconfig failed", true);
       tt->did_ifconfig = true;