Replying to Shulchanhashalem, I think that proof that the OH was the mainstream posek is that many of his pesokim is what was done by mainstream yidden in Europe, as opposed to things in MB which are being taken on in the last few decades.
For example – I know from a resident of Krakow that they would use a “hackmesser” on Shabbos, something the OH allows and the MB forbids. I believe that there are many examples of this.
Whilst the CC may have been considered the godol hador (and definitely in his latter years when he was the last of his generation of gedolim due to his advanced age), the OH was among the poskei hador (including the Maharsham and Ben Ish Chai for sefardim).
The Chazon Ish was very much for the Mishna Berura, writing that it is like Sanhedrin Milishkas Hagoziz, and his influence on the increasingly influential Yeshiva community has influenced the increased usage of the MB.
Does anyone know why there is no published OH on some parts of Shulchan Oruch? In his introduction he writes that he is publishing on all 4 chalokim, yet some parts (eg Yayin Nesech and Hilchos Avodo Zoro) are unfortunately not available. Did he never write them or are they lost?