not sure about that, i just tried attacking a malakim unit on a desert hill, and it got +40% as per its nomad promotion (with 0% general terrain modifier, since -25% from desert standard and +25% hill standard cancelled out)
however, it seems features (like floodplains and oasis) override and remove the negative terrain defense modifiers (though they add with positive terrain defense modifiers, the negative onces act as tho they dont exist).
so, because floodplains and oasis have 0% extra def standard, and deserts have -25%, the features override it to make 0%
ive tested it and gave oasis +10%, and desert oasis ends up +10%, completely ignoring the -25% of desert.
also, features cancel the double movement of the nomad trait on features like oasis and floodplains, though not hills.
id recommend giving floodplains -25% defense as well, and give the nomad promotion double movement on floodplains and oasis (let oasis remain at 0% since then its like a mini fort in the deserts, maybe even bump up to +10%).