I'll admit it. I didn't know what ohnologs were until today. That's what I get for preferring microbiology to the study of more complex organisms.
Oh, what's an ohnolog? Let's start with an ortholog: that's a gene sequence, seen in more than one species, but at one point in time it existed in a single form in the ancestor to those species. Orthologs shouldn't be confused with other similarly-named terms like homologs or paralogs, though even seasoned evolutionary biologists make the mistake every so often. No, the words "homolog" and "paralog" aren't orthologs unless their ancestral form is just "log". Don't worry about it too much.
Ohnologs are the result of whole genome duplication. They're named for Japanese-American evolutionary biologist Susumu Ohno,* who is responsible for the idea that our vertebrate ancestors (going back about 500 million years, so not just our ancestors but those of every vertebrate on Earth) experienced two rounds of whole genome duplication. Any gene duplication event creates opportunities for the new copies to diverge in sequence and function, so imagine what could happen when a full genome undergoes duplication.
The Singh lab at the Institut Curie maintains a database of ohnologs called OHNOLOGS, perhaps as the result of a terminology duplication event. They now have a paper in PLOS Computational Biology about the subject and their work as well (cited below). It's worth reading if you'd like to know why orthologs are worth studying (spoiler: many of them have functions sensitive to cancer-causing disruption) or if you'd like to see an example of a six-way Venn diagram.
Citation: Singh PP, Arora J, Isambert H (2015) Identification of Ohnolog Genes Originating from Whole Genome Duplication in Early Vertebrates, Based on Synteny Comparison across Multiple Genomes. PLoS Comput Biol 11(7): e1004394. doi:10.1371/journal.pcbi.1004394.
*Ohno is also widely credited with coining the unfortunate and inaccurate term "junk DNA", though the term itself may have predated him and may have meant many different things.**
** Dan Graur is angry about many things and this is one of them.