Besides that the bulk of the fork calls are just a preamble to starting up another process and exiting the current one. It's mostly a hack to ensure continuity for stdin/stdout/stderr and some other resources.
What usually happens in practice is you're almost OOM, and one of the processes running in the system writes to a page shared with another process, forcing the system to start good ol' OOM killer.
Sorry, but no, it can't happen, you can not fork a process and end up with twice the memory requirements just because of the fork. What you can do is to simply allocate more memory than you were using before and keep writing.
The OOM killer is a nasty hack, it essentially moves the decision about what stays and what goes to a process that is making calls way above its pay grade, but overcommit and OOM go hand in hand.