I am an Arts student (English Lit) and I find the content comparable to that of A-level. The content covered is broader, but the essays on Classics and English are strikingly similar to their A-level counterparts having studied both.
The year 1 content is denser, when I was studying history at A-level, I had one textbook with everything in it. For the first year of my lit degree, I have eight textbooks around 300 pages each and I have numerous set texts.
All OU degrees tend to begin with A-level standard work, they progress rapidly to undergraduate level. Next week, I am expected to complete what would have taken a month at A-level, in a week (The period of Stalinism). That is half of my workload as it is only my set weekly work for one of my 60 credit modules. The initial module of any OU degree starts off fairly straightforward but advances rapidly. Your second level 1 unit should be significantly harder (hence why they recommend you don't study them together initially).
The OU bypasses entry requirements by utilizing the ungraded first year to ground everyone. It provides a basis for specialization. Years 2 and 3 are comparable to that of a traditional brick uni degree. Year 1 in traditional universities is not graded at all, year 2 and 3 carry the weight of the degree in its entirety.
The OU has to meet a certain rigor, otherwise, it would lose its charter to award degrees.
If the difficulty is the only thing holding you back from finishing the degree, why not buy a year 2 textbook? I bought mine and the content is tough, it is more like a learning cliff.
If you intend on leaving the OU, completing a FM A-level and then joining a brick university to take on a degree, you might want to look at this:
https://www.timeshighereducation.com/world-university-rankings/2018/world-ranking#!/page/1/length/25/locations/GB/sort_by/rank/sort_order/asc/cols/statsThe OU is currently in 12th place in the UK rankings for young universities.