Strictly speaking, with OPOL (One Person One Language) you should only be speaking Czech with your DS. This is however only one method of bringing up your child bilingualy.
Another option is to do the OPOL method and have a family language. This is most commmonly the least used language, ie. if you are living in England then make Czech your family language as this will be the one that the DC hears least often.
In my family we have been doing it all wrong, according to the theory.
I am Scottish, DH German and we live in Germany. I speak English with the DCs when I remember but also a fair bit of German. We speak German as a family.
DD is 6yo and quite fluent in both languages, although she favours German. DS is 4yo and speaks more German but is starting now to answer in English sometimes.
We are moving to Geneva next week so we will be adding French to the mix.
You really have to go with the method that you feel works best for your family. I read a very good book on bilingualism, I will have a look at the name in a moment.
I have a Czech friend here in Germany, I can ask her if she brought her DD up speaking Czech, I am not sure.