According to one report, there was only Gyokushou originally. And out of awareness, it seem to have come to use Oushou. So, there is no difference between Oushou and Gyokushou substantially. However, higher class player usually uses a Oushou and another player uses a Gyokushou. Because of this, it seems that a player with the title uses Oushou and a challenger uses Gyokushou at the professional title match. They don't decide which shogi piece players use by the first move and the sedond move. And There is a case to use two Gyokushou called Sou-Gyoku no koma.

They decide the first move and the second move by Furikoma.

How to Furikoma

1) Pick up five pieces of Fu(pawn).
2) Put shogi pieces in both hands and shake them.
3) Throw shogi pieces on a Japanese tatami mat (Japanese floor board knit with Igusa grass) or on a shogi board.
4) When many pieces appeared the front "Fu", the player who did Furikoma becomes the first move.
5) When many pieces appeared the back "Tokin", the player who did Furikoma becomes the second move.