I tried to change settings etc. But i still got oom.
I read a great article by a great author who found the main OOM maker.
Its the setting which control the autogen distance.
I solved my oom issue, please follow these steps below.
1. Remove the high resolution, version of the scenery, and reinstall the standard version.
2. Open fsx.cfg !Remember backup!
3. Find the LOD_RADIUS string.
4. It should be set to "4.5"
5. Make you first try, it worked for me, i have PMDG 777, REX 4 4096 clouds, UTX, GSX, 4096 texture max load.
6. Try to set the setting to "3.5".
7. If you still get oom, before you takeoff.
8. Then try to lower it again "3.0, 2.5" etc.
Hope it works for you. I did not detected any quality change with my eyes, changing this setting.

Theory:
FSX is an old game from 2007. Since it´s realease and it´s 2 service packs, then still things has been developed, and developed, the developments for it, are getting never and better, but the game is still getting older. Installing FlyTampa Copenhagen on fresh fsx without using any single addon, will work very fine, with high settings. But the oom problem begin to happen, when we install all these other things, REX, UTX, GSX etc, and the highest quality plane PMDG.
FSX is an 32-bit application, and the application simply can´t handle all these things, when there is too much.
There is nothing to do. That´s why im looking forward to Prepar3D 64-bit.