The series will pick up the magical girl arc from the original series.
The extra series is being produced to thank fans for making the first series a success, according to the magazine.
A release date is yet to be announced, but the last volume of the Negima OAD is slated for Aug. 20 of this year, with vol. 31 of the tankoban release.
For a trailer for the OAD series, click here: (Negima OAD)