Walk all instances (SNMPv2c):

snmpwalk -v2c -c public <target> 1.3.6.1.2.1.14.12
snmpwalk -v2c -c public <target> OSPF-MIB::ospfMulticastExtensions

Get a specific instance (index 1):

snmpget -v2c -c public <target> 1.3.6.1.2.1.14.12.1
snmpget -v2c -c public <target> OSPF-MIB::ospfMulticastExtensions.1

Set instance 1 (SNMPv2c):

snmpset -v2c -c private <target> 1.3.6.1.2.1.14.12.1 i <value>

SNMPv3 example:

snmpget -v3 -l authPriv -u snmpv3-user -a SHA -A "AuthPassword1" -x AES -X "PrivPassword1" <target> ospfMulticastExtensions.1