Depends on a few things.
If the region A disk doesn't have any region coding on it (which quite a few don't) then it will just play.
If it does have region coding and the machine is currently set to region B then you will need to change it to region A before it will play.
This tends to be the normal procedure for a region change (but it will depend on the mod);
1. Power on the player without any disc inserted. The screen will after a while show “No Disc”.
2. About 4 seconds after the “No Disc” displayed, press DIMMER, DIMMER, DIMMER, MUTE and then the region code. 1 for A, 2 for B, and 3 for C.
3. Power off your player. When started up again, the regioncode has changed.
I have a blue ray that I bought from amazon uk sold to me as all region when I put the disc in the oppo 103d that I have the display shows a map of the world with three colour options we are in the green section B but I cant get my oppo to play it any help would be appreciated