I am studying for BA English Lit and then BSc Maths with the OU. Currently on the second module of the English and have to say it is very different to my experience at a 'brick' university.
There is some support available if you need it and ask for it (the tutors are good), but there is no 'handholding' whatsoever (in contrast to my experience at 'brick' uni, where we were told specifically what, when and how to study at each point). You get the course materials at the start of each module and just work your way through the assignments by the deadline (then sit any exams).
How much 'harder' it is depends very much on whether you prefer continuous guidance or feel happy enough working by yourself. You do occasionally meet up with other students if you attend the optional tutorials or practical lessons and I personally have not found the lack of contact a problem. But I am in my mid-20s now and I certainly would have found this method of study difficult/impossible a few years ago - mainly due to lack of confidence/motivation.
I studied English at A Level and the mark scheme for the OU level 1 module (60pt) seems to roughly match. Although the referencing is more rigorous and essays are generally longer than at A Level as you progress through the module. I studied Business Economics at the 'brick' uni, so can't really compare the mark schemes there. The essays at the 'brick' uni seemed a lot easier, but I think that was due to the amount of help we had from classes/lectures.
There is quite a jump from the level 1 to level 2 modules and although I am studying both together, I am having to put in more hours a week for the level 2 module. The level 2 module also counts as 5% more than the level 1 module in terms of your overall degree score. Judging by my experience so far it would seem that OU level 1 modules are GCSE Level content to year 1 degree content, OU level 2 modules are year 1 to year 2 degree content and OU level 3 modules are the same as year 3 degree content. So a steeper climb than a regular uni, but helpful if you intend to complete a full degree and lack (or need to revise) previous experience in the subject.