I was very interested to see you say that about research @YGRAM. My children are grown up now (and fully bilingual) but back in the day I once got kicked off a bilingual parenting forum for saying that perhaps there could be more than one way to achieve your goal.
A lot of that stuff was clearly anecdotal: one family had tried OPOL and it worked for them, so that became "OPOL is the only way to achieve bilingualism". A bit like if your child had done Kumon and got top grades in maths and your conclusion was "Kumon is the only way to get top grades".
Did a quick internet trawl now and found Annick DeHouwer's 2007 study which builds on a sample of 1899 families (sounds reassuringly large). Her findings seem more open: there is no guarantee that OPOL will lead to results, but equally it is perfectly possible to get results other ways.
This is my own experience: I have 2 children, born respectively in 1996 and 2000. I am the minority language speaker, my dh the majority language speaker. There were no language clubs or even other children speaking my language locally, but we were able to go to my country twice a year for our holidays. I speak the majority language (English) very fluently, dh speaks my language haltingly and with grammatical errors but is quite capable of communicating and reads it without effort.
There were reasons why I knew strict OPOL wouldn't work for us. One was the social situation: I did host a lot of toddler groups and other things where I needed to interact with dc and a monolingual majority-speaking child at the same time, so no way of tricking dc into believing I couldn't do this.
I also knew that I had a wealth of majority-language nursery rhymes, songs, stories, which dh didn't have in his own language. When they started school I was better placed to help them with their homework. Dd soon developed an interest in drama and that was something I could help her with and dh couldn't (couldn't scan an iambic pentameter to save his life). So I didn't want them to miss out on things I knew I could contribute. At the same time, I very much wanted them to have that in my language too. So I just put the hours in! And spent money on books, DVDs etc etc. And spoke my language a lot.
Those holidays also made it possible to create a temporarily monolingual situation in my language. There were always young cousins around who hadn't started learning languages at school and my parents always spoke our language to our dc- and even to dh.
Other than that, we were very lax about who was to speak what when. We had a vague general rule that grown-ups don't mix languages in the same sentence, but that was about it. We often had conversations that started in one language, switched to another, and then went back again. What we wanted to avoid was a sense of constraint around speaking. I did sometimes keep an eye on conversations and make sure I seized an opportunity to talk in my own language about a topic that only seemed to get covered in the other language, but that was as far as it went- there were no constraints on them.
We also talked a lot about languages to dc when they were little, using the names of the actual languages rather than "mummy's language" and "daddy's language". We wanted them to feel from the start that languages are rooted in a wider societal context, It used to be their party trick when they were about 2 to be given a word in one language and asked what it was in the other. You don't have to leave them to figure everything out for themselves- some things you can tell them.
Children's literature was great as it gave a wider frame of reference than I could provide on my own. As dc grew up, they used the internet for the same purpose. Dd watches television series in her minority language, keeps in touch with friends and relatives over social media and actually has a stronger local accent than I do.