If this is a design time pc/laptop, you're probably looking at 16GB to run them all at once, or selectively then probably 8GB.
2-3GB for the OS
1.5GB for Jdev
1.5-2GB for WLS admin server
2GB for WLS OSB server
0.5-1GB for XE
(roughly 9GB so far...)
Remember that this is enough to install and get them running.
It does not account for a production runtime usecase.
That will depend on your application requirements, design, payload size, desired throughput, component usage, etc
If you want all these apps to be up at the same time then 16 GB is recommended.