1. what does o’sullivan suggest americans should be able to possess by right of manifest destiny? 2. what reasons does he give?

Here are the answers to the given questions above.
1. John O'Sullivan suggests that Americans should be able to possess by right of Manifest Destiny is to overspread and to acquire the entirety of the continent which the Almighty has provided them for the development of the great experiment of liberty and the democratic government invested to them. 
2. The reason he gives is that it is for the full expansion of its principle and destiny of growth.
Hope this answer helps.