We only speak German at home (both dh and I as it's our mother tongue) but live in the UK. Our children attend an English school.
I try to read to them in German a lot and we have German DVDs/videos, story CDs etc.
However, we do have English books, DVDs and videos as well. I never ever speak to my children in English, only German, but I do read English books to them as I feel they should get to know them as well (and even though they do get read to at school I think it wouldn't be enough variety).
So far we only had one version, eg Bob the B. only in English, Julia Donaldson books only in English, some particular German characters only in German.
I have to say though that they're really interested to hear Bob the B. speak German (or even other language such as Italian, French, even American English, lol) as well. I can see this might not be consistent with the OPOL approach (which doesn't apply in our case anyway, here we have the family language German, the "outside language" is English). But it might to let them realise that Bob the B. (i.e. other people) can be bilingual (or trilingual or more) as well.
Also, I practise their reading with them, so they read their English Oxford Reading Tree books (and my 7-year-old his Chapter books) to me.
Talking about the books is mainly in German though.
My 7-year-old can now read simple texts in German as well although we haven't practised this a lot, only very occasionally.
I've now ordered a German textbook that's also used in German schools for them.
For my children it would be too much to learn to read in both languages at the same time, so we stuck to English first. However, I know some very clever children who learn both at the same time and don't even need much practice.