Here's a basic setup in pfSense:
Go go Services and then Dynamic DNS.
Check the "Enable" checkbox in the Dynamic DNS header.
Service Type: DNS-O-Matic
Hostname: all.dnsomatic.com
MX: <leave blank="">Wildcard: Check
Username: <your dns-o-matic="" username="">Password:</your></leave>
Click save.
See the DNS-O-Matic documentation for more details.
A DynDNS tip
If you have multiple hostnames mapped to the same IP address with DynDNS and you're using DNS-O-Matic to keep them up to date, list all of the hostnames as a comma-separated list in the host field of a single service entry in DNS-O-Matic. For example, create a single DynDNS service entry in DNS-O-Matic, enter your DynDNS username and password, and then your list of hosts: myhost1.homeip.net,myhost2.dyndns.net,myhost3.dyndns.net. I have found this to be much more reliable than creating a separate service entry for each hostname because it updates all of those hosts at once instead of one at a time. This is specific to the DynDNS update API.