My son sat the St Olaves test last November. Before I begin it is worth noting that it is searingly difficult to get in. Based on practice papers my sons scores placed him in the top 1-5% nationally and he did not get in. A large proportion of places go to children from top prep schools in the surrounding area. It is a given that you must be one of the brightest childen - after that preparation is everything. That said it is certainly possible to pass and I wish I knew then what I now know
There are 3 papers; maths, reasoning (a mix of VR and NVR) and english (I think it is use of english and a composition). If you attend and open day you can get a set of sample papers however my son said the actual papers were MUCH harder.
The papers are marked out of 100 and the children are ranked according to scores. The year before last the top score was 260 and the bottom cut off score after waiting list offers was 200 but there is almost no movement in the waiting list as most people will rank it 1st.
My estimation is that this is one of the hardest tests around and my son confirmed this ( he also sat Wilsons test and four very difficult independents). I would say that the standard of the maths test takes them up to GCSE standard to stretch out the field. I would also recommend that you focus on the composition as you can pick up valuable extra marks there as it is usually a weakness for many boys. Ensure that your son can work accurately and at speed as most boys do not finish the reasoning - it is designed this way.
Finally I would say go to the open day and really make sure that you feel your son would be happy and thrive there. With hindsight I'm not certain it was the right place for my son. It is extrememly competitive and I would not like to be one of the children at the bottom of the field as it could detroy the confidence of a bright child.
I wish you luck and hope your son enjoys the preparation involved.