Suppose Anne has a condition like OCD, that has a significant genetic component. Neither of Anne's parents have OCD, though. Does this mean that each of Anne's OCD genes were carried by at least one of her parents?
Many things are possible theoretically. Consider the following hypothetical example for illustration. Imagine, if you had two genetic variants A and B affecting a trait with the following probabilities
AA: 5% chance of having OCD
AB: 10% chance of having OCD
BB: 20% chance of having OCD
In this case, clearly having OCD has a genetic component, and yet no matter which genes a person has (and its parents had), it is possible for that person to get OCD. Probabilistically, however, they have increased risk if they inherit certain genetic variants over others.
In practice, most common psychatric disorders tend to be highly polygenic. I have answered a similar question (Q: Bipolar disorder genetics) of yours which may be relevant again.