Normally no oil painting gets framed under glass, but yes, it can be done. However this type of picture frame is not constructed very often and for a few good reasons. Firstly, the glass should not rest 
or touch even an "old", or aged, oil painting because the white oxides in the paint may take up to several months to thoroughly dry. Often, an oil painting may feel hard to the touch and yet the inside layers are still soft and curing, or drying. The painting media (plural, as there may be several, coats or paint layers of different paints) and the method applied by the artist will affect the longevity of an oil painting.
The common painting techniques requires applying several layers of oil pigments. These are often thinned (diluted) in several method, as preferred by the artist, to achieve the desired colours, hue, strength, intensity and brilliance. The paint oils have to "breathe" in order to properly dry and covering them with glass slows this down considerably.
Worse, wet or semi-dry oils may sweat .. and stick to the glass. If this happens, the oil paints may break or flake off and be damaged. Secondly, the reflection of the glass is often deemed to detract from the paint's texture. Palette work will look "flat" and such work may look more like prints under glass than oil paintings.
Thirdly, glazing a recently painted painting, even if properly spaced, will result in paint fumes staining the inside of the glass. Lastly, oil paintings, especially commissioned ones painted by professional painters, are usually already varnished and therefore, glazing the picture frame to protect the art isn't necessary. Varnish is a reversible, removable layer that stops dirt, pollutants and grime from embedding on the surface cracks and valleys of an oil painting.
On this subject, customers sometimes read up on varnishing, decide to varnish oil paintings themselves, buy some wood, furniture or acrylic varnish and promptly proceed to ruin their piece of art.This is because oil and acrylic dry at different rates. This type of varnish is unsuitable it will crack and flake. Taking this topic further, some artists also finish their paintings with special, gloss, clear acrylic polymer media such as Tri-art and Liquitex.
They do this because they consider this as insulation barrier that goes before treating the painting with a spirit-based varnish. These modern media also impart to the oil painting, a matte, semi-gloss or full gloss surface finish. However if and when an oil painting really needs glazing, , properly preparing with spacers picture frames for framing paintings under glass may add 25~40% to the cost of picture framing. This makes the job more expensive so it's requested less frequently.
That said, after an oil painting has dried and cured properly, generally a few months, it may be safely glazed. The most important task for the picture framer is to build or insert appropriate spacing devices, beads, spacers or other suitable materials (usually inside the picture frame moulding's rabbet) high enough to stop the glass surface from resting or touching the artwork itself. For costing general custom picture framing work you can get quick, easy and fast price estimates using our Custom Picture Framing Prices Estimator. Thank you for reading this post Can I ask to have my oil painting framed under glass?".
What about using the magic 3m spray an artists friend of mine mentioned might work in picture frames as it’s supposed to make stuff waterproof. I can’t find much info in the web of from suppliers. Would that fix the problem I wonder. I also heard it or some other spray can hold pastels together otherwise the colour chalks fall when the picture frame is upright.
Heh, heh .. this is actually a very old argument. I just came across a book I found in Google books discussing the merits or demerits of putting glass over oil painting picture frames. The era? 1885. Apparently the French impressionists themselves discussed this glazing option and technique. Remarkably, the pros and cons were much the same as now, 130 years later. For my opinion, don’t glaze oil paintings! When you put glass (even if done properly, on dry oils, with spacers) on an oil painting it loses its texture and vitality. In other words, it just looks like a print. That’s why I avoid glass with mine.
Another alternative to putting it under glass is to just stretch it. Most framers will be this very inexpensively and will look very minimalist and neat on your walls if done properly. Just be careful of, as the article said, any cracking or flaking because of dryness. Some framers may outright refuse to touch if it starts deteriorating like so because the stress stretching places may just make it worse.