We do OPOL. We live in England and DH is English. I speak the minority language (Dutch). DH doesn't speak Dutch and doesn't understand much either, so we speak English to each other. DS is 21mo and doesn't speak much yet, but I wouldn't be surprised if he knows a LOT more Dutch words than DH by now!
I always speak Dutch to DS, even if we're amongst people who don't speak it (I don't actually know any Dutch people here!). So at a playgroup, nursery, at a friend's house, in any company etc I would always speak Dutch to DS. Then if I'm talking to someone else in that situation I would just switch to English.
I have no worries about DS picking up/speaking English, so as an example if we're at a friend's house and she gives him some toast, I'd encourage DS to say thank you in Dutch. I will then thank her myself in English. When we leave I encourage him to say bye in Dutch etc etc. As I said, DS doesn't speak much yet so it's hard to see how this is working, but I'm not particularly worried. As DS starts speaking more I am sure he will pick up when it is appropriate to speak English (i.e. when someone is addressing him in English!). The only thing I find difficult is when I'm around other kids that are DS's age, as I automatically start speaking Dutch to them 
The few words DS does say tend to be English, but I always reply in Dutch. I don't correct him or anything (as he's not saying anything wrong! Just a different language), but I will reinforce the Dutch. So if he points to the juice and says 'juice', I will say in Dutch: "Yes that's right, that's juice".
I was in line at the supermarket recently and there was a father with his son behind me. The son was speaking English, and the father was just consistently responding in French. It sounded very natural.
My opinion is that as we live in the UK, we don't know any Dutch people here and DH and I speak English to each other, I don't need to make any effort to make sure DS will speak English. I do however need to make the effort to ensure he can understand/speak Dutch, so any opportunity for it I will take.