Is there a way to change a country's opinion of me from defensive to friendly? I'm France, allied to Bohemia and with nearly maxxed out relation (+185), they remain defensive (they won't answer a call against their enemy Austria, their opinion of the Austrians is at -4 and I even get a -40 "their relations to the enemy" penalty on the call -_-
First off, that's about their attitude, not their opinion. Attitudes are partially derived from opinion, but it's not the same thing. Defensive is not an attitude, so I'm not sure what their attitude towards you is. Either way, having high relations like that and fighting lots of wars together is pretty much all you can do in the hopes of changing their attitude towards you to friendly.
Regardless, you're not getting a penalty because of their attitude towards you, you're getting a penalty because of their attitude towards Austria. Apparently Bohemia doesn't really have that bad an attitude towards Austria in that game. I wouldn't be surprised if they're simply neutral towards them with an opinion like that. You said they're enemies, but in in-game terms that just means Austria rivaled them, doesn't stop Bohemia from being indifferent towards Austria. You can find out what exactly their attitude is by going to the opinion mapmode, selecting an Austrian province and hovering over Bohemia.
Unfortunately, it is quite hard to change this attitude without giving one of them provinces that used to belong to the other. You could possibly work around it though. If Austria is the emperor, you could find any HRE nation that Bohemia will join against and attack that.