Sorry if this is a repost or if its in the wrong place … wasnt too sure about where to put it.
Many people say the thing you dream of is the last thing you think about before falling asleep , is this the case?
If not is there any way to ensure you dream of a certain thing or at least promote an idea that will increase the chances of you dreaming of it ?
I think it depends how much effort you put into thinking about that certain thing.
I’m not sure if you dream about the last thing you think of, I don’t think it’s true unless you’re putting a significant amount of effort to thinking it.
Sounds like you’re asking about VILD, where you would usually imagine a dream scene where you perform a reality check, several times a day. However, if you’re not interested in becoming Lucid, then you could just imagine whatever scene you have in mind several times a day and hopefully you’ll end up dreaming about it.
I disagree on that. My dreams seem to be a product of what I’ve experienced during the day, or the previous days. If i remember correctly you start dreaming about 4 hours after you fall asleep at night. You also have many dreams per night which mostly got different content. So I think it’s unlikely we dream about the last thing we think of. However, if i wake up during the night and instantly go to sleep again, my mind starts to wander and eventually form dreams. This is what WILD is. But I often do it unconsciously.
That being said, once you turn lucid you can manipulate the dream to dream what you want 
And for me, doing something a lot during the day increases the likelihood of me dreaming about that, lucid or non-lucid.
That’s been said many times but it doesn’t hold true to me. It’d be cool, though. I guess it does to other people - so it seems it’s personal.
If I WILD or DEILD, unless I already have vivid imagery going in my head that I can immerse myself in, I usually don’t end up in a dream and instead find myself in a black area. It’s almost always like an OBE, and I find myself in my house and bedroom. It’s pretty annoying, since I often find it very hard to leave our property.
What exactly causes OBEs? Is there a scientific Explanation - like they are caused by imiganiation or are they a spiritual thing or what?
I’m not too sure. I’ve only ever had one and it was just extremely strange.
It was also first thing before I went to sleep (So it wasn’t a dream…), I just found myself instead of being in my body hovering in the air in my room, while also hearing noise from the other room. As soon as I realised, i fell and suddenly i was back in my body. I could also still hear the noise too when i woke up. This was like 10PM before bed… 
No idea how I induced it, it just happened, but it was not dreaming.
I don’t think much research have been done on the subject. Here is a website i discovered long time ago that I haven’t really checked out but it may be a starting point for you: oberf.org/
Some people says the Pineal Gland has something do with OBEs. David Wilcock is one of them. I watched this presentation by him years ago and he mentions OBEs somewhere in it https://www.youtube.com/watch?v=Y5b-kLvppdg. I don’t remember exactly where he talks about it but the presentation was generally interesting.
There’s all sorts of explanations, ranging from scientific (pineal gland, for example) to spiritual, and even to thinking an OBE could be a lucid dream or the other way around. I guess technically a dream is an OBE, because you certainly isn’t in your body lying in bed all night long