All monosaccharides and disaccharide dissolve in water. Why is this?

All monosaccharides are soluble in water

All mono and disaccharides have multiple -OH groups.

These are able to easily form multiple hydrogen bonds with water, allowing them to be quite soluble.

Are oligosaccharides soluble in water

The question is too generic to answer.

Oligosaccharides are too big a group.

Some are soluble, some arenu2019t.

Some are sweet, some arenu2019t.

Some are reducing some arenu2019t.