Might need a bit more info. Oleanders don't mind it being hot and dry, they still need copious watering in the summer though. Could it have been seriously dried out....or over watered ? What about red spider mite....they love Oleanders and show by having little webs under the leaves or turning the leaves mottled.

have you fed the plant ? they will appreciate a weekly feed in the growing period.

If its just one leaf, well things happen and there is really no cause for concern from my point of view of course unless your plant has a really low total leaf count.