True but not relevant. This isn't a privacy thing, its just purely about accessing pirate bay and other blocked sites. So open source and so on is irrelevant in that goal.

Edit: Quote added. "“This browser is just to circumvent censorship, to remove limits on accessing sites governments don’t want you to know about,” The Pirate Bay notes."

Does the Opera license allow to distribute modified versions setup to automatically use Tor for .onion URLs, without getting special permissions from Opera Software?

In addition: Why Opera? I don't see any significant advantages over Firefox. There are probably more users who are familiar with Firefox than with Opera.

Also, being open source, one can evaluate the source code and check the "secure browser" isn't leaking secrets.