Yes, it's pretty much all open, with the exception of a small number of closed-source, third-party add-ons such as the nvidia graphics drivers. And most distributions either keep those in separate sections of their repositories or refuse to distribute them at all.
In the end though it all depends on the license
each individual source file was released under. Your common "Linux" distribution is really a collection of separate projects cobbled together from around the FOSS community, centered around the Linux kernel and the GNU core utility toolkit. Other major projects usually included or available are xorg, gnome, kde, mozilla, samba, and many more, none of which are exclusive to Linux.
The vast majority of FOSS projects are released under the GPL (either solely or as part of a dual-license), which was created specifically to protect the "Four Freedoms
" of users. Much of the rest uses even looser licenses such as the BSD
, which are pretty much just a step short of public domain.
Some technologies, such as the mpeg group codecs, also have patent and other IP considerations to consider, however.