This is an interesting psychological tip that is more effective than you may think.
Rushing Hero's Mansion and annexing an Oasis early in the game shows everyone around you that you mean business.
This works quite well when facing novice or intermediate players, they will see the oasis and think "Okay, I'll just leave this guy and attack the other one".
Pro players may take advantage of it by attacking the oasis, but I rarely experienced that in early stages of the server.
+you do get the resources production bonus
COM 3X - Phreaky