"The O" was a virus that mysteriously appeared in Barnaby City in the summer of 2015. The origin of the disease remained unknown, and it persisted for about a week before being eradicated by the efforts of Encore. Those infected by the virus were healed, yet some still retain permanent damage both physically and mentally.

Those infected with The O were referred to as "Odes".

The O manifested as elongated, torus-shaped objects of varying sizes and consistencies. The surfaces were shiny and oily, with a slimy, gelatinous texture, yet they maintained a solid form, akin to a non-Newtonian fluid like oobleck. The O was largely odorless, except for a faint scent reminiscent of burning plastic. Its taste was uncertain, as perceptions ranged from positive to negative, influenced by the state of infection of the individual at the time.

It is unknown if the O was sentient, but it exhibited positive chemotropism toward nearby entities and manipulated the behavior of those it infected.

The virus primarily spread through physical touch, inhalation, or consumption, indiscriminately infecting targets regardless of their sentience.