This a problem that only occurs with the 4GB Patch installed - OMSI can still run out of memory even with it installed, it just has issues like this rather than throwing error messages.
The fix is for bus developers to use DDS rather than PNG for their textures, and in particular to use smaller textures, especially for interior parts.
For comparison: a 4K 24-bit bitmap texture uses 64 MB of texture memory, but reducing it to 2K means it uses only 16 MB. It follows that reducing it further to 1K would mean it uses only 4 MB, and so on. If the 2K texture was instead a PNG texture, it would use 21.4 MB (regardless of bit depth), but as a DDS texture it would use only 16 MB even though all variants have some form of transparency support.
LOD meshes or configuring a bus to unload unimportant meshes when operating as AI also help a lot as unloading meshes also means the textures used on them can be unloaded. A grey blob can suffice for a dashboard in an AI bus, allowing all the dashboard models, textures and scripts to be unloaded.