The first question about the disk is a question for your system or VM administrator.
Second questions answer is YES as per Oracle recommendations you need for 4 cores 2 for OMS and 2 for repository. It has been seen Java is a CPU hogger in OMS. You should be watching your Java's CPU and memory consumption. You also need to keep this is in mind that it's a VMware means its sagred resource and depends on how the resources allocated to the box. Your system admin can watch this for you.
The missing 12 GB will come from the swap file I think. If you run the server using Windows you won't see this file by default.
If you look at the hardware requirements ( http://docs.oracle.com/cd/E24628_01/install.121/e22624/preinstall_req_hw.htm )
you will see you need about 20 GB for the OMS and about 50 GB for the repository. The OS needs some GB and the database is probably running in archive mode so the archives are also stored on disk. Conclusion: 100GB is not too much to have as disk space. Ask the Administrator to increase the disk to 200GB