*Lets keep this as spoilerfree as possible*
In short : whan I've done OWB main quest, can I still do the sidequests? Please answer yes or no if possible.

Yeah, you can go back and fourth from the Mojave Wasteland after the MQ in OWB is done...

Ok. Good, thx!

That doesn't answer the question. The question is if you can complete side quests after completing the main quest. My guess is no, but I am not sure.

All you can do is try it and see what happens. While yes we can go back to BT that doesn't mean we can complete any side quests that we didn't acomplish before.