How to make oil paint very matte

If you do not mind the very fast dry time, my first thought was the Gamblin Matte alkyd colors. A matte appearance in oil paint is usually tied to a very leanly bound paint or one that is so dispersed that the granularity of the pigments is apparent, refracting the light and creating a matte and undersaturated appearance. There is a fine line between matte and underbound. Many works of the early modernists are underbound because the artists drained oil from their paints and freely diluted them with solvent. That is all well and good for works that are very valued and are cared for by conservators in museums. This is not a good situation for the average artists.
I generally caution against adding large amounts of wax to oil paints as it completely changes its solubility. Such works are certainly viable, but they should be thought of as encaustics rather than oil paints. Make sure to record your working process on such paintings so that future conservators know what to expect.
There are likely amendments that can be added to paint which can impart a matte quality without sacrificing film strength. I have never sought such an effect in oil paint. so I have little experience with this. I am sure that others here can advise better than I can. I will send this to a few of them.

​As Brian has stated, a matte appearance in oil paint typically indicates that the texture of a grainy paint component is apparent in the paint finish. The conditions which induce this effect usually suggest underbound paint, the presence of excessive, oil-absorbing fillers, or a stripped top envelope of oil (bare pigment on top of film). That said, a desire to manipulate surface sheen is perfectly understandable, and (to a point) not all matte finishes indicate a defect or paint failure. 

Adding absorbent fillers is one strategy for matting oil paint, but too much can lead to a weak film. One of my teachers who I would consider an excellent “paint mechanic” once discussed using diatomaceous earth as a matting agent and filler. He theorized that the spiny, velcro-like texture of micro-fossil seashells would interlock within a paint film, improving integrity. I think this might be worth trying.

Wax, as Brian mentions, is problematic because of solubility. If there is enough wax to produce a dry, matte finish, it means that wax granules are heavily present in the top layer. Varnish and cleaning solvents would easily soften and intermix with the paint, in this case.

Pure Gum Spirits of Turpentine, when used as an oil paint diluent, is very effective at producing a matte surface. When paint vehicle is thinned and viscosity is reduced to the point where pigment envelopment is affected, loose and underbound particles can be present on the top of the film. I think it is possible to use turp. for matting oil paint without complete technical failure, but if the dry film yields residue when rubbed with a cloth, obviously that is a problem. 
Matte varnishes are a good option for imparting a uniform, low-shine surface, but varnish is not normally considered a permanent part of the finished art. Matte varnish is good for reducing shine in difficult lighting situations, and it affords protection which bare, matte paint lacks. Artists who want a flat, fresco-like surface typical of many Modernist paintings are probably not going to be satisfied with matte varnish. If the artist wants matte varnish to remain a permanent specification, that needs to be documented on the object.

Manual manipulation of the dry or semi-dry paint surface can be an option for artists who want a durable matte paint surface. Rubbing with a cloth and mineral spirits can remove some of the top glaze and expose some dry solids without destroying deeper film integrity. This still leaves the paint  less protected and harder to maintain, but it’s my personal first choice.

​Matte paint surfaces are the result of light scaterring due to surface texture, whcih may be the result of underbound paint (not sufficient oil to envelop pigment particles) or large pigment particles or both.

As Brian commented there are amendments that increase the matteness of an oil paint film. These include large particle extender pigments, such as calcite (chalk or marble), kaolinite, silica, talc, etc. These amendments can be added as powders directly to oil paint, or mixed with oil and then added as a paste to oil colors.

When adding powder to your paint, it will thicken considerably, so a small amount of oil may be added to offset this effect. In addition, you can add an excessive amount of powder with the result of underbound paint, which would not be good for the longevity of the painting.

Painting on absorbent grounds will cause sinking in of oil which also results in matte appearance of oil paint.

​Thanks Matthew and George.