This term refers to oils, acrylics, and paintings on board that have been varnished, so protection by glazing is no longer necessary. This means that a mount isn’t necessary either. Of course, if you want to add a mount for the visual effect, by all means do so, but you will need to glaze it. Sometimes a small oil can look better treated in this way.
The width and colour of the frame is even more important without a mount. A narrow frame on an oil looks mean and amateurish; even on small oils. You can combine two or more mouldings to achieve the desired width, and the inside frame can act as a brightener, or a way of defining the edges when there is a danger of the colours of the painting merging into the frame in places.
If you don’t want the frame to overpower the painting, choose the colour carefully. Complementary colours (those opposite each other on the colour wheel, like yellow and purple, blue and orange, and red and green) accentuate each other. Putting a reddish gold frame on a painting with a blue sky, for example, is wrong, in my opinion. Silver, grey or white would be better, in general.