They can get by on very, very little water--they're all over the freeway medians here where we get no rain at all during the summer, so if it's anything I'd guess too much water, I think you'd be hard pressed to underwater them unless you live in the desert and even then it would take a while. I'm assuming these are plants that have been in the ground for a while since you didn't mention that you had just planted them, but if they're newly planted then it's probably transplant shock.
Too much water is the better case scenario, but the worst case is that it could be oleander leaf scorch. It's a big problem in southern CA, and it's also been found in AZ and TX. It's a bacterial disease that's spread by insects, glassy-winged sharpshooters are one thing that spreads it but there may be others. And unfortunately there's no cure, best thing to do if this is the problem is get rid of the plant ASAP and thoroughly sterilize anything that you've used to prune/cut it. If there's any possibility this is what it is, I'd take a piece to your local extension office and see if they can tell you whether it's the leaf scorch or not. Here's some info on the disease: http://www.cnr.berkeley.edu/xylella/oleander.html
Hi mctraill, best way to check for watering problems is to stick your finger into the soil, if dry about an inch down, the soil is too dry, the opp for too much water, if a large plant, then scrape some soil away and test that way, dont say what kind of soil you have, but it may be it aint holding onto the neutrients, feed, that a lot of plants need, so maybe it also needs some liquid plant food to green up the leaves, are there any other plants suffering from the same probs, that would be an indication if neutrients are a prob as others would be suffering too, worth a look, no feed will work instantly though so you may have to wait a few days for any pick up to show, my plant grows indoors in UK and it shows simmilar signs, but usually a good water and feed helps first, but Ecrane also has a valid point about bugs carrying deseases, so really need to check out all posibilities over the next few days, it could be in your heat, you have a crust on your soil and watering aint reaching the roots of the plant, so look for all these signs, Good Luck, WeeNel.
Oleanders can get by without much fertilizer, so while feeding the probably wouldn't hurt them, I don't think that's what is causing yellowing leaves. If it's a newly planted plant, then the heat could be stressing them or it could be that it's not getting enough water, but if it's an established plant they are very heat and drought tolerant, so it's much more likely in that case that it's either too much water or it's the oleander leaf scorch. Hopefully not the scorch since there's no cure for it!
When I lived in Phoenix, my Oleanders lived on the side of an irrigation ditch, which means every two weeks they'd get a deeeeeeepp soaking for over 24 hours. They grew like weeds. No fertilizer, horrible soil. Had to have them trimmed by 2-3' every year.
Do *not* fuss over these plants. But you'll want to look around your neighborhood for any others that look like they're dying to rule out the scorch... like everyone else said, lol!