You can simple negate a condition by using a not. Following condition is a true, when both of values are not a x.

{
  not: [condition1, condition2, ...] // negate(condition1 AND condition2 AND ...)
}
{
  not: condition1 // negate(condition1)
}
{
  fields: [{
    name: 'Or condition',
    component: 'text-field',
    condition: {
      not: [
        {
          when: ['a'],
          is: 'x'
        }, {
          when: ['b'],
          is: 'x'
        }
      ]
    }
  }]
}

As the value you can use an array (AND) or another condition.