Gregarious among grass and mosses in meadows, unfertilized lawns and open grassland, lincluding dunes and dry coastal slopes. Autumn. Widespread and fairly common in most of the area; occasional but not very common in Norway.

Pileus 5-20 mm across, conical or parabolical, translucent-striate, sulcate, pruinose, glabrescent, somewhat lubricous, hygrophanous, greyish to grey-brown or dark brown, often with a yellowish or olivaceous tint, or with a pinkish or reddish shade at the centre. Lamellae 13-20 reaching the stipe, ascending, narrowly adnate, dorsally intervenose with age, whitish to grey to olivaceous grey, the edge yellowish brown, pink-violet or reddish brown, sometimes not coloured or somewhat coloured only very close to the margin of the pileus. Stipe 20-80 x 1-2 mm, hollow, equal, terete, straight, pruinose at the apex, glabrous farther down, whitish, pale grey or olive yellow at the apex, gradually becoming darker below towards the grey-brown, sometimes dark brown base, the base densely covered with long, white fibrils. Odour nitrous or indistinctive. Taste +/- raphanoid.

Basidia 24-40 x 7-11 μm, clavate, 4-spored (some seen with 2 sterigmata), with sterigmata 3-7 μm. Spores 9-11(-13) x 4.5-7, Q 1.7-2.2, Qav ~1.9, pip-shaped, smooth, amyloid. Cheilocystidia 30-80 x 8-20 μm, forming a sterile band, with reddish brown contents, fusiform, clavate to somewhat irregularly shaped, long- to short stalked, smooth or covered with few, rather coarse, simple to furcate excrescences up to 27 x 2-3.5 μm. Pleurocystidia not observed. Lamellar trama dextrinoid. Hyphae of the pileipellis 2-4.5 μm wide, covered with simple to very much branched excrescences 2.5-30 x 1-2 μm, forming dense masses. Hyphae of the cortical layer of the stipe 2-3.5 μm wide, sparsely covered with cylindrical, straight to curved excrescences 2-5.5 x 1.5-2.5 μm, terminal cells rarely seen, diverticulate, not inflated. Clamp connections present at all tissues.

M. olivaceomarginata is a very variable species, which may appear in a number of colour forms. Some of them have been considered to represent separate species or varieties. Maas Geesteranus (1986 c) referred to four different forms, but in my opinion there are so many intermediate and overlapping collections that they should not be formally separated. It has also been suggested (Arnolds 1982, Ludwig 2012) that it is conspecific with M. citrinomarginata, a view that I find very unlikely. However, ITS sequences from the Norwegian Barcoding Project (NorBOL) do not seem to distinguish between the two species.

In spite of the great variety of colurs in M. olivaceomarginata, identification rarely causes any trouble in the field. The terrestrial growth should distinguish it from resembling species. M. citrinomarginata, also a variable species with colours from very pale to fairly dark, can be told apart from M. olivaceomarginata as follows (according to Maas Geesteranus 1986 c):

• M. citrinomarginata can be found in forests but it seems to be absent from the dry grasslands, where M. olivaceomarginata is most common.
  
  
• M. citrinomarginata generally is paler and more yellow, and the lamellar edge is citrine yellow to whitish, while it has one or another variety of a brown colour in M. olivaceomarginata.
  
  
• The terminal cells of the stipitipellis in M. citrinomarginata are numerous and covered with coarse excrescences while they are very rare, appearing absent in M. olivaceomarginata.