The ostiomeatal complex is a collective term encompassing the maxillary sinus ostia, the ethmoidal infundibulum, the hiatus semilunaris, the middle meatus, the frontal recess, the uncinate process and the bulla ethmoidalis.  The following diagram shows a CT scan of the ostiomeatal complex and a line drawing of the ostiomeatal complex to the right.  Locate the terms in white on this diagram.
 

Clinically, the paranasal sinuses can be divided into two groups, an anterior and a posterior group, depending on the association between the sinus and middle turbinate.

The anterior group includes the frontal, maxillary and anterior ethmoid sinuses.  These drain near the infundibulum.

The posterior group includes the sphenoid and posterior ethmoid sinuses.  These drain into ostia situated above the attachment for the middle turbinate.

The ostiomeatal complex represents the relationship between the middle meatus and osita of the anterior group of paranasal sinuses, in particular the anterior ethmoidal cells.

Look at the following diagram, it should make this concept easier to understand.
 

If there is an anatomical abnormality e.g. concha bullosa, or a disease process that brings the two muscosal layers into direct contact then localised ciliary stasis occurs, free sinus drainage is obstructed and sinus infection occurs.

Ostiomeatal obstruction caused by mucosal contact points or bony abnormalities inhibits mucociliary contact points and produces chronic mucosal changes.

Anatomical obstructions include:

• Concha bullosa
• Septal deviatin
• Haller cell
• Rotated uncinate
• Paradoxical middle turbinate
• Scar tissue