The OOM message is a very well known issue, and it doesn't have anything to do with the power of your PC or the memory you have. FSX, being a 32 bit app, is limited to use an absolute maximum of 4GB of RAM, regardless how much memory you have.
Those 4GB are further limited because, as a DirectX 9 app, some of this memory is taken away by the video card too so, it's not even 4GB, but more likely 3.5-3.6 GB in total.
This means, there's no single cause for OOM, but it's the COMBINATION of all your installed addons. The most memory-consuming item, is very likely the memory hungry airplane you are probably using. And of course, if you use JFK with a big scenery like New York City X, this won't help.
Your options are:
- Lower your settings
- Don't use too many sceneries at the same time
- Don't use a memory hungry airplane
- Switch to DirectX 10, that will result in some savings that might just save you from an OOM
- Switch to FSX:Steam edition, which is a little bit more optimized in memory consumption.
- Switch to Prepar3D, which is a DirectX 11 app (but still 32 bit...)
