This offertoire is one of the more complex pieces in this volume to play. Long legato lines in four part harmony are a nice challenge for 10 fingers. It's structure on the other hand is simple. The first half is simply repeated in the second half, transport a quarter up and a short 8 bar coda finishes the piece.
Score available here: http://partitura.org/index.php/ernest-grosjean-offertoire-bref-4