Stage 4 Engineering student here. It varies from tutor to tutor but many of my modules have generally had tutorials starting at around 6/7pm and running for an hour or two. A few tutors did tutorials on a Saturday and one did 2pm to 3pm.
How it works is that each module website has a tutorials tab on it, where the tutorials available for that module are all listed. within each will be a selection of session slots done by different tutors. For some, you might be restricted to just your tutor group, while most others will let you sit in on other tutors tutorials so you have a decent variety of options available. The usually also mark which ones will be recorded, so you can often catch a recording a bit later.
Most of the time, each section of a module (a block) will have an associated tutorial where the tutor will recap/ summarise the subject and dive into main elements as well as give pointers where needed on what to focus on for an assignment. Beyond that, you study the material and work on in-material activities and such.
You can, of course, contact your tutor via email if you have additional questions or are struggling and they usually are happy to arrange a 1 to 1 call to go over the problems. Forums are available for general guidance too. Many students on modules band together in a whatsapp group too, so you can get an element of social engagement with your fellow sufferers.... I mean students.
Welcome to the OU community, it really is a great place to be and I wish you luck. Happy to help where I can with any questions.