Hi,
As MAC address will be auto assigned by OpenNebula according to the MAC prefix + IP address, there are cases where i need to define the MAC for a particular VM - software linked to the MAC address.
So, how do i insert the MAC Address for a particular VM?
maybe 2 quick ways.
1.- set a new net with only 1 ip and custom mac [1]
AR=[
TYPE = “IP4”,
IP = “10.0.0.150”,
MAC = “02:00:0a:00:00:96”,
SIZE = “1”,
]
And second way… ovbiate the mac at context and set the custom MAC in eth config. at file /etc/sysconfig/network-scripts/ifcfg-eth0 (for redhat based) you can add the var: HWADDR=XX:XX:XX:XX:XX:XX
if you not use mac as var in context file (or cloud-init) you can set custom.
I know isnt the best way, but it works! 
(hope developers at Opennebula give you a right solution)
1: Managing Virtual Networks — OpenNebula 4.10.2 documentation
OK, i found it. We can define the mac address when add the lease using CLI instead Sunstone.
onevnet addleases <vnetid> <ip> <mac>
Some users are facing these similar Mac-based systems problems to change the default IP address. To overcome these issues can take advice from Mac Outlook Search Not Working that would provide fruitful guidance.
Open System Preferences, click on Network and then click on the Advanced button. Make sure to select the appropriate interface first (WiFi, Ethernet, etc) in the listbox on the left. Click on the Hardware tab and you will see the first line is MAC Address.