Offset overhand bend

The offset overhand bend (OOB, ABoK No. 1410) is a knot used to join two ropes together. The offset overhand bend is formed by holding two rope ends next to each other and tying an overhand knot in them as if they were a single line. Due to its common use in several fields, this bend has become known by many names, such as thumb knot, openhand knot, one-sided overhand knot or flat overhand bend (FOB), though the terms "one-sided" and "flat" are considered incorrect.
Uses.
Long used by weavers to join the ends of yarn, the offset water knot is very old. It was one of the knots likely identified among the possessions of Ötzi the Iceman, who dates from 3300 BC. 
The knot is also tied in a slipped form by mechanical balers to bind straw and hay, but this bend is not practical to use as a binding knot when tied by hand.
In climbing and mountaineering.
In rock climbing, the offset water knot is a favored knot for joining two ropes for a rappel longer than half the length of the ropes. 
There is controversy over its safety, as it can fail by capsizing under high loads, and some American climbers refer to it as the European death knot, abbreviated to EDK, with some sources recommending against its use. Failure of this knot has been implicated in some accidents and near-misses.
Many sources argue that this is a misnomer, and the knot is safe for abseiling and rappelling, since this does not generate such high forces, and the knot, being on one side of the twin lines used in abseil, sees only half of this force. They believe that with proper attention given to dressing and cinching the knot, the risk of capsizing is highly unlikely.
Several sources recommend adding a second overhand as close as possible to the first (a "stacked overhand" or "double overhand") for most situations, which maintains most of the benefits of the single overhand, while preventing it from capsizing. Another option is wrapping the strands a second time before passing the tail through (also called a "doubled overhand").
Easily formed in most lines, the offset overhand bend is jam resistant at nominal loads of one person (approximately 100kg). The jamming threshold is thought to be approximately 3kN (300kg). The instability threshold is thought to be approximately 5kN (500kg) – that is, a capsizing event becomes increasingly probable as loads approach 500kg. It is critically important to pay close attention to dressing and cinching of the knot before attempting to abseil. That is, climbers must exercise due diligence when tying this knot – by pulling firmly on each of the four rope segments – which is necessary to achieve a properly compacted and cinched dressing state.
Despite questions about this knot's security, it does present some advantages for use in rappels. Because the knot is offset from the "axis of tension", it can translate more easily over rough surfaces and 90 degree edges than other knots; and it is quickly tied and readily untied. Since a stuck rope on a descent also represents a serious hazard to climbers, these advantages, along with ease of tying, have led to its popularity. It is recommended by some sources with the caveats that the tails be of sufficient minimum length (never less than 200mm), the knot be diligently dressed and fully tightened by pulling individually on all four rope segments, and then subjected only to moderate rappelling loads.
Furthermore, the knot can be rotated to induce a choking effect to trap and crush the tails. Virtually all testers appear to only examine this knot in its mid-rotation state. It is theorized that this "mid-rotation state" is in fact the orientation where the structure is most vulnerable to capsizing. In addition, when tying the offset overhand bend using different rope diameters, the thinner diameter rope must be positioned underneath the larger diameter rope. This tactic further inhibits any likelihood of capsizing.
The offset figure-eight bend, a similar knot using the figure-eight knot, has been used in the belief that its greater size and complexity brings more security. But testing and more than one fatal failure indicate the figure-eight variant to be less secure, more prone to capsize at lower loads, and in capsizing uses more of the ends than does a capsizing overhand bend. Moreover, while there is one proper dressing of the overhand bend, there are a couple of dressings for the offset figure eight bend.