Every time you wash the oil away it produces more. Most people wash their undreaded hair every day. If they were to wash it every seven days instead, it would get a bit greasy and prolly itch a little. In a couple of weeks the hair will adjust to this new schedule of washing and produce less oil. It would no longer be oily or itchy.
Dreaded hair works the same way.
When your dreads are new be sure not to use real hot water cause you don't want to melt out all the wax until they have a chance to tighten up more. New dreads will loosen a bit when you wash them especially if you don't use a nylon. You will have to backcomb them a little after you wash to tighten them up. Before you backcomb you can use the locking accelerator. This will make the backcombing a lot more effective.