Obsessive-compulsive disorder and major depressive disorder are among the most common mental disorders globally and are frequently co-diagnosed. They are both highly complex, heterogenous and a result of both environmental and genetic factors. This review focuses on the genetics behind these disorders and their overlap. When researching OCD, the genes that have most commonly been associated with the disorder are ones involved in glutamate, serotonin, and dopamine transmission. Such genes include SLC1A1, DLGAP3 and DRD4. In MDD genetics, genes related to neurotransmission, like TPH2, are also relevant but so are genes involved in several other biological pathways, like BDNF, LHPP, SIRT1 and NEGR1 which are involved in mitochondrial biogenesis, cell adhesion or neuronal survival. When researching the overlap between OCD and MDD, common mechanisms, shared genes and treatment effects must all be taken into consideration. Besides irregularities in neurotransmission, reduced amygdala and hippocampal volumes have also been observed in both OCD and MDD patients, as well as abnormalities in the regulation of the HPA axis. That implies shared genes between the two disorders may be involved in those mechanisms. 5- HTTLRP, a functional polymorphism of the SLC6A4 gene has most consistently been associated with both disorders and it affects the serotonin transport. Another gene relevant to both OCD and MDD involved in the same pathway is HTR2A, while FKBP5 is a gene affecting HPA axis regulation and shows great promise in researching the overlap of these disorders. The shared treatment effects, such as SSRIs and neuromodulation therapy, further confirm the idea of shared genes. These and future findings about the genetic overlap between OCD and MDD will help develop better and more effective treatment for people suffering from both these disorders.