Unfortunately, OSCAD is particularly demanding when it comes to waterproofing. Since OSCAD does not support curved surfaces, we have to replace such surfaces with a mesh of polygons. It is absolutely critical to get the correct orientation of every single polygon. I have tried to write a check and repair routine. It examines each polygon and reverses the direction if it has the impression of an orientation error.
Nevertheless, it happened to me again and again that OSCAD rejected individual bodies as "not watertight". And I did not manage (despite great efforts) to find the gap. And I have not been able to get help through the forum.
That was a few months ago, maybe the situation has improved in the meantime.