I keep getting configurations of M,N and O that are different from the model answer. I have no idea where I went wrong and I can't seem to make sense of the mark scheme. Any help would be much appreciated!

The answers I've got: M : (R, R)  N: (R,S) O: (S,S)  

The mark scheme:

 

 

For M: Carbon 1 has the aldehyde; carbon 2 has the methyl group. Carbon 1 priorities: 1) C-Br 2) C-C/Br/CH3 3) C-CHO - I get an S; carbon 2 priorities: 1) C-Br 2) C-C/Br/CHO 3) C-CH3 - I get an S (it looks R but the H is in front so reverse to S).

For N: same priorities as M. Carbon 1 is R; and carbon 2 is R.

For O: same priorities as M. Carbon 1 is R; and carbon 2 is S.

For P: same priorities as M. Carbon 1 is R; and carbon 2 is R. 

 

I think where you're making the mistake is that you're making the aldehyde the second priority and the C-C-Br group third priority; however, the C-C-Br group is second priority. Think of playing the card game war. 

 

I have an A, you have an A. Go to the next card. I have a Q, you have a K. I win. Does this help?

For O, you also need to be aware of drawing conventions with Fischer projections. Namely, the bonds on the left and right are coming out towards you.