My experience:
I don’t know how long OI has been a significant contributory factor in my ME. From the start I have needed to lie down completely flat, for example when I was still working full time, I found if I was able to lie on the office floor in the middle of the day (not always an option) for ten or twenty minutes my day would be less exhausting. I found I was able over time to get more brazen about, though other people get worried about it, if I needed to lie on the floor in ‘public’. When a relapse forced me to give up work, I was aware that lying totally flat on my back on a firm surface was physically the most restful position, even if not always the most comfortable option.
It was only about ten years into my ME I consciously noticed a hierarchy with leaning less tiring than standing unsupported, but that sitting was better, further sitting with my head supported was even better, lying with head raised better still, but that lying totally flat provided the most rest. However I assumed this related somehow to physical effort.
Further over time trying to do anything bending forward became noticeably more tiring, though I had no explanation for that, other than some half backed idea that being overweight there was some how physical pressure on my heart or restriction placed on my lungs. (Added - I have not discussed orthostatic issues with a doctor though I did have my heart checked when my ‘chostochondritis’ was very marked, with the chest pain being very severe. I do now have high blood pressure, but I am nearly seventy, unfit and overweight.)
Initially in the course of my ME I spent time researching the condition, but after a number of years going through the trying everything and anything to mitigate the condition, I recognised that there was nothing I could do and just dropped out of any groups or following ME news and just got on with doing whatever the current course of my ME allowed. About fifteen years into my ME I got more involved in online groups and forums and became aware of the concept of orthostatic intolerance, which was a better explanation of my experience than just being upright requiring more physical effort.
It was only with the most recent and most disabling relapse, some twenty years into my ME and some ten years ago, that I was only able to stand for five to ten minutes without getting dizzy and breathless and at risk of fainting if I tried to push it. It was then more obvious that being upright placed a greater restriction on what I could do than the actual physical exertion involved. Then I was very consciously aware of being upright as something I needed to minimise in contrast to the previous idea that being totally flat was most restful position.
Being pretty much bedbound, I then had a high backed chair by the window so I could watch the birds on the garden feeders sitting with my head supported in short bursts, also I would try standing looking out of the window. However, though I could very slowly do both for longer over a number of years, I can not say if this was an incremental reconditioning or a reflection of a very gradual spontaneous improvement in my underlying ME.
Now, though I still can’t do anything requiring bending forward such as weeding, which within one or two minutes results in dizziness and breathless and risk of fainting, I can be upright for several hours at a time. I have more or less returned to being flat as good for resting rather than actively avoiding being upright, though still needing to average being horizontal for between sixteen or eighteen hours a day to avoid PEM. However when in PEM being upright again become more obviously difficult, something to avoid.
Speculation:
I suspect that, perhaps every one with ME/CFS has some degree of orthostatic intolerance that contributes towards the exertion required in doing anything upright, such that being horizontal is the most restful position for us, however that this does not necessarily involve any obvious adverse effects being obvious whilst standing. The possibility of reduced cerebral blood flow does not seem an unreasonable explanatory option.
However there is a second stage where being upright produces much more obvious adverse effects, possibly including POTS, making standing something we actively need to avoid. I don’t know if this relates to reduced cerebral profusion or if a second mechanism would need to be invoked.
[edited to correct some typos]
which might be helpful 
.