It looks like your Oleander has gall disease. This is a common problem in California and other parts of the West. Keep the foliage of the plants dry so avoid overhead watering. Clip out all infected branch tips and remember that Oleander is highly toxic so wear gloves and do not burn the cuttings as the smoke can be an irritant. Damp weather in spring can cause this problem so you may not see it next year if the weather is dry. Keep growing, Marianne Binetti