How does OEC explain the existence of pre-fall death? To make things clear for everyone, OEC=old earth creationist (i.e. God used evolution to make his creation over billions of years). Most people arguing against OEC pose the seemingly irrefutable claim: 

Life couldn't have evolved because this implies the death and suffering of millions of organisms, and death and suffering were introduced into this world at the same time that sin was introduced, and sin was introduced only when Adam and Eve ate of the forbidden apple.

I am not an OEC myself, but I have tried to think of a possible answer to this without success. My best shot was that Adam, Eve, and the tree of knowledge were fictional too, but I believe God makes it clear that sin entered the world at a specific time. So What is the typical OEC answer to this claim? Thanks.
I'd encourage you to give a listen to William Lane Craigs 'Defenders' podcast series on Creation and Evolution, specifically episode 14 which touches on this (but the prior 13 episodes provide background info). This is probably the most erudite, methodical, and approachable analysis that I've ever come across on this subject.

There are several issues touched on here. What does it mean for a non-human animal to suffer? Do non sentient beings suffer in the same sense that we do? What about animals that have some self awareness? There is a lot philosophical/ethical writing on this topic. The answer is, is that although animal undoubtedly feel pain, fear and anxiety, you do not necessarily have to hold that animals suffer in the same way that humans do (having a self awareness of their own suffering).

Also, it is hard to believe that many of the creatures that exist today would have been able to exist in an environment where no death or suffering was intended. Predators come to mind; parasites and virus as well. 

Finally and most importantly, how you define death? When death enters into the world does it mean death in the physical sense? Or does death mean corruption and in the spiritual sense, that humans and creatures are not living to their true potential in harmony and communion with God? Indeed, when Adam and Eve sinned, they did not immediately die in any physical sense which is evident in the text itself, but they were immediately separated from God. In other words does sin=death=physical death for humans and all creatures? The answer if you hold to evolutionary theism is that the author of Geneses, and Paul (or anyone else) when he references these things in the NT, is that they mean spiritual death. If you read 1 John closely, he is using death and sin interchangeably, and the meaning is extremely spiritual in its use.

So an OEC could believe that although the account of creation is primary metaphorical and doesn't describe a physical mechanism by which creation occurred, they can believe that Adam and Eve where historical characters (as you mention) and that they were in communion with God but when they sinned, death in the spiritual sense entered into the world, separating humans from God.