Mount Erymanthos (Greek: Ερύμανθος, Latin: Erymanthus) overall is an irregular massif of peaks connected by ridges embedded in the mountains located in the north of the Peloponnese, Greece. Erymanthos is on the west side. Its highest peak, Olenos or Olonos (Ωλενός or Ωλονός), Olenos original and preferred, elevation 2,224 m (7,297 ft), is often called Mount Erymanthus, and conversely, Mount Olenos can be used for the entire range, although the customary usage is Erymanthos for the range and Olenos for the peak.

Olenos is not a single peak. The topography close-up depicts a three-peak complex, three main peaks, of course, as any terrain presents many bumps under various descriptions. The summit is a N-S directed ridge with Olenos on the north, and another peak, Granitis, elevation 2,192 m (7,192 ft), 32 m (105 ft) lower, on the south. Olenos is at 37.988197°N 21.83422°E / 37.988197; 21.83422. Granitis is at 37.986724°N 21.834143°E / 37.986724; 21.834143. The horizontal difference between peaks is 166 m (182 yd). The key col of Granitis is 38 m (42 yd) to the NE. The prominence of Granitis is 4 m (13 ft). Many superficial maps confuse Granitis and Olenos.

It being clear that between any two peaks there is a key col, a topographical island is defined as the terrain above the plane of the two closed contour lines tangent at the key col. Olenos and Granitis are an island. If the summit of one closed contour line is greater than the summit over the tangent closed contour line, the greater summit is the parent, or encirclement parent, of the other. Thus Olenos is the parent peak of Granitis. To find a parent of Olenos in the Peloponnesos, however, one must travel all the way to Mount Kyllini in Korinthia, 49.6 km (30.8 mi) to the east, which has an elevation of 2,374 m (7,789 ft). The key col is 20.5 km (12.7 mi) ESE.

There is a 3rd peak, which is not the highest of the range, nor is it traditionally Erymanthos, and yet it has inherited the name, Erymanthos, perhaps because it hangs over and is an essential part of the Erymanthos River watershed. Location is 37.982599°N 21.848584°E / 37.982599; 21.848584, elevation 2,146 m (7,041 ft), prominence 315 m (1,033 ft). This Mount Erymanthos is visible from below at 37.948106°N 21.854954°E / 37.948106; 21.854954 on the left side of the road in the village of Platanitsa, Achaea. The ridge of Olenos is visible above Erymanthos to the right, and the upper Erymanthos river, here a trickle in an otherwise dry Canyon, can be seen on the right. The village has a lot of plane trees but is on the edge of the tree line.

Olenos is the fourth-tallest peak in the Peloponnese peninsula. The summit is on the boundary of Achaea and Elis. Historically, Mount Erymanthos was part of northwestern Arcadia, where it was the second-tallest mountain range. The northern portion belonged to the historic Achaea.

The name, Erymanthos, is a legacy from prehistoric times; that is, the Bronze Age, and possibly before. It does appear as an implied region in Linear B, the script of the Late Bronze Age. Whether it is Indo-European or not is debatable and has not been resolved. It is not Greek. Olenos, on the other hand, is Greek for ulna, "forearm," although the reason for the name has been lost (some sort of bend has been suggested).

In contrast to most of the other mountain ranges of Eurasia, Erymanthos seems to have no discernible geometric pattern, which leads to certain difficulties in discerning which peaks are in it and which not. In the literature the borders are somewhat flexible; one source will state that a certain village or mountain is in, while another refers to it as out. More recently (1992 and after) the problem of definition was confronted by the Greek government in compliance to the European Union's Habitats Directive of 1992 requiring its member states to establish Protected areas in the Natura 2000 network.