A phobia is a fear of a specific object, event, or situation and the resulting attempts to avoid the feared thing.
This can sound similar to OCD, and people often confuse the two disorders.
Many people will say that they are afraid of flying. If people can get on a plane after doing some rituals that they feel will make them safe and that will counter the obsessions that they have about flying (an image of the plane falling from the sky), then they may have OCD.
However, if they have a flying phobia, there is not much that they can do to make the fear go away (without some therapy or self-help assistance), and they avoid flying at all costs, even if it means losing their job, then that is a specific phobia. Specific phobias are typically limited to one thing, whereas OCD is usually more general.
Individuals with a flying phobia may have no problems with any other type of travel, whereas people whose OCD is focused on travel will have obsessions and compulsions about any mode of transport.