If you need to re-encode: $ ffmpeg -i input -c:v mpeg2video -b:v 12M -pix_fmt yuv422p -c:a pcm_s16le -map 0 output.mxf
Should result in OP-1a. Note that, depending on your input, you may have to declare additional options such as an output frame rate with "-r" (or the fps video filter). Default GOP size will be 12, I believe, but this can be changed with the "-g" option.
For encoder capabilities/restrictions and a list of private options see: