Ime (2 bilingual children, aged 19 and 22), it is not actually about this method or that; it's about finding a way to ensure what I feel are the essentials to become fluent in any language:
frequent and regular exposure
pleasant associations with the language
circumstances that force you to use the language at least from time to time
For many people, this is what OPOL does, particularly when it comes to the last item on the list. For others, MLAH does the same thing.
Dh is English, I am Swedish. We did not stick to OPOL or any other fixed method, but I used Swedish a lot not only in conversation, also story telling/ songs/nursery rhymes/games etc. We were also able to visit family twice a year, which made a big difference.
The fact that I also used English didn't confuse them or bother them: by the time they were old enough to go to school they already knew I could speak to their friends and their friends' parents and that I was actually teaching English literature, so there was never a chance of pretending I couldn't do this. They also knew that dh could speak Swedish and heard him do so with my family.
What I think we did manage to establish this way was that being bilingual is a normal, everyday thing. They also heard me speak French and German when travelling and they knew the house was full of books in different languages.
I talk a lot, so the exposure thing was not a problem.
The pleasant associations bit was covered by story-telling and games, nursery rhymes, fine collection of books and DVDs (mostly gifted by family members). Dd soon became an avid reader, so that was further exposure.
And using minority language was enforced when we visited my family, some of whom were very young and clearly did not speak English.
Both children are likely to start by speaking Swedish when they phone me up, though we may then switch backwards and forwards several times during the course of a conversation.