We don't have enough information to help you. Oleanders are full sun plants when grown in the hot, tropical/sub-tropical locations in which they are happiest. They require excellent drainage and once established, are considered a low water requirement plant.

I suspect that your plant is potted in a poor mix of some kind, and it might be that you aren't watering it sufficiently at any given time. It should be drenched thoroughly and then allowed to dry out until you drench it again. Then again, the plant may stay soggy for too long (for a variety of reasons) and the roots have begun to die.

Both of those scenarios will cause wilting.

The oleander needs full sun and is pretty easy to grow. If you think you are losing it, for something other than disease, cut a couple of branches and root them. They are easy to root.. We root several a year and give away. Unfortunately, this year, I got my reds and yellows mixed and don't know who got what. I have two left that the wife has been on me to put out. She wants them put at the edge of my garden and when they bloom and you can see what color they are, I will have to transplant again. I don't know where you live, but in SC they do not do too well west of the Midlands.

I'm in rural Sumter County, SC and I have oleanders growing in in a flood plain that's soggy during winter. I planted them 6 years ago, before hearing oleanders don't like wet feet. Mine are extremely cold hardy, and stayed fully green during winter 2011's extreme cold. My flood plain oleanders are 8'x8', and are still blooming like crazy. They do root easily, but make sure your cuttings have new growth at their tips.

Sounds like it needs to be repotted, roots trimmed. Classic conditions of being rootbound.

Your soil is not holding on to the water when you do water--probably too peaty of a mx. Once peat soils dry, they resist hydration. They should be outside, in full sun, all day. I would chuck it out of its pot, root prune a bit and work it into a larger container with a more humusy, but well-draining soil. If the soil drains, they are water lovers. I had the same problem with mine and that's what I had to do to save them. Good luck!