I am no expert, Rogi, and I don't know what the weather is like in your bit of Germany, but ours haven't really started into proper spring growth yet, despite the spring-like temperatures, and I would try potting them on now rather than waiting, and giving them a good feed to encourage them on their way. I've had to prune very hard as the severe and prolonged cold has killed most of the branches, but the new shoots which it put on in late summer are looking quite good. Ours are planted in the ground, by the way. My reasoning is, if you leave them in the pots, they won't have so good an opportunity to grow and develop so well during the summer. Ours continue to flower for a long time.
I shall be very interested to see what the experts advise you to do.

I'd do it now, no point in waiting till its flowered, that happens later anyway.
Gattina, as a point of interest, I have one planted in a small south facing garden.immediately behind a 2 foot wall, here in West London - its about 4 feet high, and has remained completely untouched by the weather this winter, so is green and leafy and looks like its budding up. The winter last year didn't wreck it either, despite the low temperatures - but then its in a good micro climate where its planted.

We have had some fearsomely low temperatures here this winter, Bamboo - it touched -22°c one night, and for a couple of weeks was around -12° day and night. Above all else, the weight of the snow (well over a metre deep) seems to have done the damage. It's pretty amazing what HAS survived, I suppose.

It's swings and roundabouts though - you probably get much more flowering there on these than we do here because of the extra heat in summer.

Thanks for all the answers. When I was down in the garden this afternoon I found a bigger pot, put some well rotted compost in the bottom and put the Oleander on that, then I filled the sides up with ordinary garden earth....now I suppose it is "just wait and see what happens".

Not a believer in proper potting compost then, Rogi?

Yes, Bamboo, however bad the winters, our oleanders give us the most beautiful, exuberant display of colour each year - happy to see the sun and feel the warmth. Let's hope this year is no different. I haven't ever had to prune them back quite so hard before, and never this early in the year.