OOM errors can be the result of many things.

Number of lights and their shadow types and whether or not they're volumetric.
Number of image maps and the sizes of them. Multiple Poser figures create problems here.
Whether or not you're using GI and/or Radiosity and/or HDRI image maps.
Number of polygons not in eco-systems
Whether or not you're using native VOB files or imported 'other' file formats.

Perhaps you can check a few of these and see if they are the problem.