You may be able to gleam some copyright knowledge from this discussion:

http://opengameart.org/comment/32206#comment-32206

I'm no legal guy, so don't take anything I say as fact.

There is a lot of grey area and debating when it comes to copyright and derivative works. Safest bet is to do everything yourself, only using other works as references, not bases. You may be liable for legal action against you if someone can prove that something you created is a direct result of copyrighted material.

Imagine you write a song and copyright it. The song isn't very popular but whatever. I then come along and make a song based on your song and pass it off as my own. My song does become very popular. If you know that I used your work and can prove it, you are going to feel cheated and will want some form of compensation.

It's different in different places of the world, but often "derivative works" are included in copyright. I remember at least one case when one artist said he was "inspired by" another artists scuplture, and so a painting that he made was consdiered a copyright violation, because it was a derivative of the sculpture, even that it was not a scuplture by istelf and completely newly made. But it is extremely complicated as a quick web search revealed (you can can google the painting and scupluture problem, seems to be a more common and even example case that I thought first).

But IANAL so I can give real advice here.

Actually, the situation described above seems to be different. There isn't enough detail in the post to tell, but it sounds as if he's made a set that is technically compatible rather than artistically derivative. Could you clarify this, please?

I've read it again and yes, it might be that I misinterpreted the "using copyrighted sprites as base" phrase. A newly created spritesheet with moves and positions compatible to the originals shouldn't be a problem, at least it is very common thing in the game modding community.

This depends on whether you plan to use the work commercially or non-commercially.

Despite popular belief, copyright law does not give copyright holders complete control of their works. As long as you are using the art in a transformative way for a non-commerical use and not blatantly distributing the art as is, then your use of the copyrighted material constitutes the "Fair Use" clause in copyright law. You are not required to attribute back to the author or contributor of the works in any way if this is the case if you do not wish to.

For example, a search engine such as google may show the images contained on this website without giving credit back to the original authors. The images might come directly from this site or a 3rd party site, but google is not breaking any copyright laws because this constitutes as Fair Use.

If you intend to use the work commercially then you have to follow the license for the work with few exceptions.

One Exception includes, but is not limted to: monatizing traffic to your content that is protected under Fair Use with the application of advertisement schemes like Google Ads.

The facts I've stated are valid, but don't take my word for it, do some research.

I have personally been in a situation where squaresoft sent me a cease and desist letter for using graphics from Chrono Trigger in a flash game back in 1998. I won my argument with the Fair Use clause as the game was considered a form of personal artistic expression and not a commercial product.

PS. Even if your use of a copyrighted work is protected by Fair Use, it is still common courtesy and good practice to cite the author of the work. They did create it after all.....

There's been at least two different artists who submitted sprites clearly based on Final Fantasy VI character sprites.  Both of them argued it was done from scratch and one even told me I don't know shit about copyright law and I should be ignored as i'm so "wrong troll", despite having those lines burned into my brain in the 90s from playing FF3 on the SNES enough to recognize it off the bat.  Both were ultimately removed.

 I do know shit that "fair use" can't work in open source licenses whether you're doing it commercially or not.  Once you put something "fair use" under an open source license, full use is granted "for any purpose". This is wrong and blatant misinformation, not to mention illegal.

Given the sudden horde of iPhone mobile game developers using OGA as their marketplace who knows what kind of litigation chain this kind of thing could end up when SquareEnix legal finds out.  Ignoring this "troll" would mean bye-bye OGA.

Well the thing is, you cant license anything out that contains copyrighted works under the Fair Use clause, since doing so violates that clause because you do not have that right under fair use. You can however license the software or code itself that uses the copyrighted material and make it open source. You can even commercially make available or open source the software that uses the copyrighted material as long as you disclose that the material is included with the software as education purposes(such as placeholder), or parody example: http://www.kongregate.com/games/ngamesltd/pockie-ninja-ii-original where wherein the creators used the likeness of characters from Bleach and Naruto in the form of a parody style anime game.

Here is also an example in which you may use copyrighted assets commericially: Game Guides/FAQ's/reviews that include images from the game in which your game guide includes. Before the popularization of internet game guides, there was a plethora of magazines that you could purchase that included game guides, FAQs/ reviews and such which used copyrighted material in the process of achieving these ends because once again, the works are used as education which is fully permitted under Fair Use. Now days, you can find these types of guides included in 3rd party apps on the google play store and others. They can sell these apps which include copyrighted artwork without breaking the law.

There are a lot of circumstances in which you are protected from legal reprocussions under the Fair Use clause which negates the need to obtain licenses free or otherwise but the license does indicate how far you can take the Fair Use clause.

For example:

Most LPC content has some sort of free license and as such people may use them if they follow the restrictions of the provided license. This being true, anyone can develop an engine that uses the LPC content and release the engine open source while using the LPC graphics as a means to educate people how to use the LPC works in the engine and disregard the most of the restrictions of the license such as citing the author of the works. The people that then download and subsequently use the engine must obtain and follow the license for those artworks if they intend to use them(unless of course, their use also constitutes Fair Use).

On the Contrary, you can not include copyrighted material in the same way in which the license does not permit free use, since educating people how to use those assets is the same as distributing unlicensed works and teaching people to use/steal them for their own use. There are some exceptions to even this as well because you may not be educating people in using those copyrighted works, but instead using those copyrighted works to demonstrate how graphics can be used in the engine generally as long as you disclose that they cannot actually use those copyrighted works and that they are for education purposes only.

Click here for more information regarding Copyright and Fair Use: http://ogc.harvard.edu/pages/copyright-and-fair-use

In conclusion, you can legally ignore most of the license restrictions on the OGA art when making games as long as it isn't for commercial use, and in some cases even if it is for commercial use, especially if the author/artist has not officially registered the artwork with a copyright office before you use it. (This makes it increasingly hard to seek legal damages and to fight the suspected violator's Fair Use defense.)

Sorry OGA, but Fair Use makes most CC licenses useless in most cases, however, if anyone uses your art and claims they made it, that is different from copyright violation as it is considered plagiarism. You can most definately take action against this. :) If you truely care about adding an extra layer of protection for your work, get it registered. It cost money but it's your art; if it's valuable to you, do it.

If someone uses your art, and you believe their use does not constitute Fair Use, but you haven't registered your copyright yet you have 4 problems:

1. You MUST register your copyright before you can take legal action in court.

2. If the person used the copyrighted work before you registered it, YOU have to prove that the violator in question is breaking copyright law and as this is an extremely gray area at this point in litigation if you're suing someone, you'll need to hire a lawyer and it will not be cheap and the defendant does not necessarily have to prove he/she did not break the law. 

3. Not having the work registered before the violator in question uses it will embolden the violators defense in court.

4. The violator in question could actually argue that he/she created the copyrighted work(he/she could register it before you) and counter sue YOU or the OGA owner(Bart) for statutory damages up to $150,000 per registered work infringed upon.

with that have been said, leaving your artwork unregistered is a huge liability for everyone involved.