Is Ubuntu an open source software (OSS)?
This is not a simple question at all.
As well as Debian’s one, official repository of Ubuntu includes free (mostly) and some non-free software in different sections, but their names are not so clear:
universe consist of free packages,
multiverse – of non-free. Some of packages in the latter two could be really necessary to use you PC (like some firmwares and some drivers), but the rest are not.
Moreover, Ubuntu makes installing non-free software easy enough to be installed accidentely without realizing what did you install.
If you are concerned about non-free software on your PC, you might use
aptitude to find out whether anything non-free is installed:
$ aptitude search '?installed (?section(restricted) | ?section(multiverse))'
or install a small utility
vrms, and ask it:
If you are conserned a lot about using only free software, you might consider installing a Trisquel GNU/Linux, which is basically completely free Ubuntu.
As for the term ‘open source’, if you just use it as a synonym to ‘free / libre’, see all the above. But if you cares about Open Source Initiative opinion and want to follow it strictly (there are actually few who do it, but if), then even the
universe sections of repository includes some software, which is not ‘open source’ according to OSI: OpenSSL is a most notable example. Yes, OpenSSL cannot be technically called ‘open source’ according to OSI (despite the http://openssl.org states the opposite), since its license is basically conjunction of license of SSLeay and 4-clause BSD License, so it inherited obnoxious BSD advertising clause, which is not OSI-approved.
Yes it is!
Ubuntu software is free. Always was, always will be. Free software gives everyone the freedom to use it however they want and share with whoever they like.
Refer the Ubuntu philosophy for detailed information.