Solo runs are a lonely and curious affair, both difficult and easy at the same time, as you accumulate six times as much experience as anyone else.
The OMA challenge aims to resolve these issues, because what's the point in being incredible if there's nobody around to see?
1: The One Man Army (aka OMA) naturally accumulates groupies on as time goes on. Whenever they encounter a potential new party member, they must accept them (and indeed, seek them out as soon possible, the first port of call in BGII must be the carnival to pick up Aerie, the OMA in BG1 must pick up Xzar, Monty and Imoen before making their way to pick up, for example, Ajantis and Shar-Teel).
2: The OMA must care for their groupies, reviving them at the earliest possible opportunity in the unlikely event of their death. The only time the OMA may remove a party member is to replace them with another party member.
3: For their part, groupies are utterly useless wastes of space. On entering a map, they must remain by the map entrance until the OMA has cleared the way (you are allowed to Ctrl-J your posse to the exit when your OMA gets there, to minimise the boring). If a map does not require the OMA to "gather their party", then there's no need for the groupies to follow.
4: Should groupies encounter enemies, they must retreat, immediately, to the nearest corner, where they will cower, waiting for the OMA to save them. At no point may a groupie carry any item that is not plot-bound to them, swing a fist, pick a lock or otherwise do anything, ever, including talking or being talked to by anyone outside of character specific plotlines. Doing so will cause the OMA to instantly die of shame and embarrassment.
4: Should any effect that would end the game in a solo encounter occur and inflict upon the OMA, the game is over. They have lost.
In effect, the OMA challenge is a solo challenge without solo advantage. They only gain one sixth of the available exp, whilst carrying the entire party by themselves and suffering from a constant financial drain if their partymate somehow dies.