I have a few potted orleaders. I brought the larger ones into a (currently) unheated (unattached) garage. I still have smaller one outside. They are fairly cold tolerant (within reason). I would suggest that you bring it in once temps regularly fall below 30 F. (remember, pots offer less cold protection than the ground). Good luck!

Oleanders are native to the tropical West Indies; thus I would protect it from 30 and below temps. Young potted oleander plants are more susceptable to extreme cold than older plants in the ground. Good Luck!

Frost is the worst enemy of Tropical plants.Protected from frost some of the tropicals can stand cold down to 30 deg. F. but not for a long time.

Oleanders are not tropical plants native to the West Indies, far from it, they are native to Spain, Portugal and across the Mediterranean Basin and into Saudi Arabia. They can take quite a beating from occasional freezing weather in habitat and also in mediterranean climates, with some of the hardiest cultivars easily taking 25F temps in winter without problems. Of course in containers, the roots will obviously be more sensitive to freeze damage.

If your plants are gradually hardened off in the fall to cold, and were not heavily fertilized late in summer, they should be fine left outside with temps above 30F, and brought into an unheated garage when it gets colder overnight. They are accustomed to winter rains in habitat, so do need some watering over winter to keep them going.

can oleanders stay in an unheated garage with little light over the winter?

What is your minimum Winter temperature in that garage? I overwinter several in an 'underheated'/detached garage. 50% of the garage door surface is glass and it faces directly South--so there is some sun and solar heat. But as long as your minimum temps in the garage are not much under 30 F., you should be okay!--(My Winter day temps in the garage average around 50 F.)

My sister who lives near London, England has growen an oleander in her garden since 1993 and it is still there flowering every year. Now it stands over 6' high. I think oleanders are really tough plants and can stand cold conditions.

It got down into the 'teens at night for a few weeks here last winter and the oleanders are fine.

I never even get damage on mine when they go down to 20F! They are hardy plants that can handle the teens for sure! I protect mine pretty well which is why they survive NYC winters (not a hardy cultivator either), but they are very easy plants and I find that they actually prefer to be outside than inside!
-Alex