OU courses do start off easy, to bring in the people with no previous qualifications and get everyone on track. The level 2 and 3 courses are equivalent to the same level at a standard university though. The grading is the same too (specifics of the marking scheme aside), so for example, high and low 2:1 answers would be graded the same elsewhere.
One thing that's different is that they operate a threshold system for allocating grades (or did a few years ago), so a certain % of people get firsts, then 2:1 etc. Lots of universities used to do this, as a way of levelling results between years, based on the very sensible idea that the difficulty of exam papers can vary a bit year to year but the average ability level of a very large cohort of students will not.
Individually, if you're borderline, that can work either way for you. Generally, it means that a certain % of people will always get firsts and 2:1s but also 2:2s and presumably 3rds. So you cannot get the grade inflation that some other universities have seen, with huge numbers of top grades.
A quirk of the OU is their demand for consistency between coursework and exams; probably another failsafe against people cheating the distance learning system. If your marks diverge, you get the lower grade, not an average. So you do need to be as steady and consistent as possible. Tutors do have some discretion about coursework deadlines though and are generally understanding about 'real life' circumstances, probably more so than in another university. So you have more freedom to get things right in your own way, according to your circumstances. But, if you can't pull it together in the end each year, it can go very wrong.
If your own experience and that in your tutor group is that everyone seems to be doing very well, that's probably to do with who turns up and speaks up. Generally, mature students are more conscientious, organised and motivated, so do the work, turn up and are keen to discuss things. That doesn't make people cleverer or more able though, it just means they'll give it their best shot and might be more likely to fulfill their potential.
Also, really check how your course is assessed and graded, before you start. Then you'll know which modules really count.