The game is 32 bit so it will only use around 4gb of ram to begin with, and because of the poor garbage collection generally (deletion of memory objects no longer needed) you will eventually run into this problem while playing the game. After long periods of time. Or even loading large areas with a lot of things.
If there isnt enough space left in the maxium of 4gb TLOPO can utilize you will get this error and crash. If you have an integrated graphics card this is even worse as some of the texture files instead of being stored in Vram will be stored in system ram, further limiting the capacity.
If you have low system ram to begin with, then you might have problems when running other programs alongside TLOPO. If you have 5 gb of ram: the OS will use maybe 500mb-1gb regardless, plus if you have a Internet browser open, you may be down to only 2-3gb of usable ram left for TLOPO.
Thats the causes of the out of memory crash anyways. Many of the crashes I epierence are not due to out of memory unless I've been playing for a while.