Oronsay was built for the Orient Line in 1925 by John Brown & Co. of Clydebank. The surviving records of this company were purchased by National Archives of Scotland and currently reside in Glasgow University Archives.
http://www.gla.ac.uk/services/archives/
But, if you are happy with a small scale model, you will find 1/1200 plans of the ship in the excellent book 'More Miniature Merchant Ships' by John Bowen. Pretty easy to obtain. This book, (and it's precusor companion volume 'Miniature Merchant Ships) is geared up to scratchbuilding projects and gives a lot of practical advice. Worth a look!
good luck!