The MJO indeed can have a reinforcment/cancellation effect on an El Nino event.  For a great intro on the MJO, check out this guest post on the ENSO Blog by Jon Gottschalck.

Depending on where the rising and sinking sections of an MJO event are located, they can be associated with an enhancement or suppression of the trade winds. In some cases, an MJO could enhance the trade winds which would be the exact opposite of what you would expect from an El Nino. Or vice versa.