From OpenNMS

Being an open-source product, support comes in two flavours: free, community-based support, or paid commercial support.

Community-based support

The free community-based support is provided by way of mailing lists and an IRC channel. The developers and many users are subscribed to the mailing lists, which get a moderate amount of daily traffic discussing and helping out with installation and configuration issues people run across. If you have OpenNMS installed, joining the mailing lists is strongly encouraged; even if you can't help out answering questions, it's not unusual to pick up helpful tidbits on ways of using and configuring OpenNMS, from what other users are trying to do. Also available is the OpenNMS IRC channel (#opennms on irc.freenode.net). This is where a lot of the developers hang out, and can be a good way of getting your question directly to one of them, especially the more complex ones which might require a bit of "back-and-forth" to get to the nub of the issue.

Paid commercial support

Commercial support and services are available from the OpenNMS Group, Inc.. They fund several full-time developers, and provide that commercial level of support and accountability that some customers require, with everything from short term installation support, to on-going configuration help, right up to 24x7 on-call support. See their website for more details on current offerings.