Not familiar with the Onslaught! games? Check out our post Onslaught! -- a history of violence (in images).
Onslaught!'s music composer Joshua Morse has released the Onslaught! OST and it's really awesome. You should absolutely go pick it up on Bandcamp right now. Need more evidence? Check out the promo video below:
We've chosen a GPL (General Public License) for Onslaught! Arena. This means you can fork it and make whatever you want, but it must also be open source. We also retain the rights to all game assets. tl;dr: make whatever you want with it but you can't sell it.
Note that only Onslaught! Arena (and not Onslaught!) is open source. While the games are just slightly different, Onslaught! was built using the Game Closure SDK and cannot be shared just yet.
One final note: if you are one of the fine human beings who purchased the game while it was for sale, we'd like to thank you from the bottom of our nerdy hearts. You've kept us going and have given us hope for chasing the indie dream. Thanks for your support!