Fantastic! (That's what Dave expected, I believe.) Mucho thanks!
Classically, often you'll see:
OSPF Adjacency Requirements,
From the perspective of OSPF, there are a couple of things that must match for a OSPF neighborship to establish; these include:
The devices must be in the same area
The devices must have the same authentication configuration
The devices must be on the same subnet
The devices hello and dead intervals must match
The devices must have matching stub flags
What Dave had described, and what MHM confirmed, is you can match authentication via area settings and/or interface settings.
Years ago, when (on Cisco devices) authentication could only be defined in the OSPF router section, and implicitly impacted all interfaces for the authenticated area, it was and all-or-nothing situation.
The only possible limitation I now see, if you configure authentication, in the OSPF router section, for all the area interfaces, you might not have a way to disable it for an interface to not use authentication, on an interface, to establish adjacency with another router not using it at all.
E.g.:
R1
interface e0
ip address 192.168.1.1 255.255.255.0
!command to disable authenication on just this interface to R2?
!i.e. not "no ip ospf authentication"
!but something like "ip no ospf authentication"
router ospf 10
area 1 authentication
network 192.168.1.0 0.0.0.255 area 1
R2
interface e0
ip address 192.168.1.2 255.255.255.0
router ospf 10
network 192.168.1.0 0.0.0.255 area 1
