Pretty much all options we have are OS dependent.
If you trust Microsoft having a copy of your BitLocker keys on the cloud, Dislocker is an option. It is a software to help you mounting and dealing with this kind of encrypted device.
Also, there is some work being sone bi libfvde to make File Vault 2 available to other OSs but, as far as i know, this software is still in its childhood.
Finally, Librecrypt can help you to access
luks volumes on Windows, but there is no solution when it comes to Mac OS being the client here.
Best you can do is to still rely on some multiplatform encryption software like TrueCrypt was before being discontinued.
The Story behind TrueCrypt:
As of TrueCrypt, much of the media coverage was cause the one of the head developers are now at Microsoft developing BitLocker(theory one), US government identified the developer and sent a National Security Letter demanding to put a backdoor into TrueCrypt(theory two)... Whatever is the reason, they don't have a real proof to point out why TrueCrypt became unsafe. Lots of speculations... Taking a look at Webcite:
And then the TrueCrypt developers were heard from . . .
Steven Barnhart (@stevebarnhart) wrote to an eMail address he had used
before and received several replies from “David.” The following
snippets were taken from a twitter conversation which then took place
between Steven Barnhart (@stevebarnhart) and Matthew Green
TrueCrypt Developer “David”: “We were happy with the audit, it didn't spark anything. We worked hard on this for 10 years, nothing
Steven Barnhart (Paraphrasing): Developer “personally” feels that fork is harmful: “The source is still available as a reference
Steven Barnhart: “I asked and it was clear from the reply that "he" believes forking's harmful because only they are really familiar
Steven Barnhart: “Also said no government contact except one time inquiring about a ‘support contract.’ ”
TrueCrypt Developer “David” said: “Bitlocker is ‘good enough’ and Windows was original ‘goal of the project.’ ”
Quoting TrueCrypt Developer David: “There is no longer interest.”
Even being an option, lets not take risks here, with security related software that suffers from speculation problems, and that are considered abandoned by the community.
VeraCrypt is the fork based on TrueCrypt 7.1a with improvements, and shall work multiplatform with client installation. License is Apache v2 and shall meet the opensource aspect of software on the question.
As portability is the question here, this software is the best you can get because even using BitLocker, luks or FileVault, all of them are widely available to just one OS Family.
Best you can do here is to find a "portable" version of VeraCrypt(and trust that software), format 2 partitions on your usb drive with the first of them
FAT32, put all portable versions(Mac, Windows, Linux) there, and make the second partition the encrypted one. That is the theory. I don't know if this will work :)