Don't go searching what is legal in commander. Search what's banned. ALL the cards are legal except those in the banlist.
September 12, 2017 6:42 a.m.
The answer provided above is correct, there is a relatively short banlist for Commander, the list can be found here
Since you just started out it's important to make a distinction between Masters sets like Modern Masters and Iconic Masters on the one hand and Standard Sets (for example Ixalan will release at the end of this month). The cards in the Masters Sets are only reprints and keep the legalities they had prior to their reprint.
On the other hand, cards that are printed in Standard legal sets are legal in standard and enter into modern this way. For new cards this isn't that difficult as they then become legal through standard in all other formats, however for reprints this is the case.
For example Opt was not modern legal, but will be printed in Ixalan, for this reason it will be legal in Standard and Modern once Ixalan releases.
I hope this helped a little.
September 12, 2017 6:55 a.m.
September 12, 2017 10:22 a.m.
Why are they banned? There's way more op cards legal