My first reaction is the yellow oleander aphid which we have here. If you have good eyes for closeup viewing, you should have seen the little yellow bugs (aphids) clustered on the stems of new growth where it is still soft and succulent. But if you need bifocals, and you weren’t wearing them, it could appear as yellow spots. Yellowing and scorch on oleander due to drought.
Oleander is not a low water use plant but is very drought tolerant, two very separate issues. The other possibility on the oleander is damage due to underwatering which will appear as yellowing and eventually browning. If that is the case just increase the watering. Oleander is not a low water use plant to look good. Although it is very drought tolerant it is a high water use plant and does very well under rather frequent deep irrigations much like any other nondesert shrub.
By Robert Morris