Experts and BHG readers answer.
Black spots and powdery mildew on Oleanders
You'll probably have to cut the oleanders back to the live, healthy growth at the base and let them regrow. It sounds like they were stressed when you bought them and didn't begin to show it until after you'd planted them in the ground. Look after them carefully this year, making sure to water them during the hot dry spells.
To choose a tree for privacy from your neighbors, use the plant finder in BHG.com's Plant Encyclopedia. Choose 'Detailed Search' and you can search by height.