Sites providing an alteration service don't violate any of the licenses accepted on OGA. They may hold a temporary copy of the asset on their servers, but there is nothing against any license condition in doing so. Even if it were, that would generally be considered fair use. It might count as a derivative, but all OGA-accepted licenses allow for creating derivatives. However, some licenses (GPL's and CC BY-SA's) require you to share-alike such derivatives, so be sure to share those changes under the same license as the original.

If the site in question is covertly taking those files uploaded and secretly using them for purposes other than what they've claimed (like distributing them under alternate license terms) that would be a potential violation, but it would be on them, not you. I've never heard of a site doing something so shady, though. So it's probably not a real concern.

If the site in question has a clear intent for redistribution (i.e.: you're uploading assets to shutterstock or other image hosting sites), that would still be ok as long as the asset is being redistributed under the same licensing terms and giving proper attribution. If you take a GPL asset from OGA, for example, then upload it to Textures.com and share it as a royalty-free license claiming someone other than the author created the asset, yes that would definitely be a copyright violation, but that's a pretty extreme example and is easy to avoid by simply sticking to the licensing terms and giving proper credit.

TL;DR: If you're talking about something from OGA, you've got nothing to worry about. Just be sure to stick to the license conditions. Let us know if you have more questions. :)

<standard disclaimer>I am not a lawyer. This should not be taken as legal advice. Licensing concerns are not subject to generalization. If legal concerns should last longer than 4 hours, consult your attorney.</standard disclaimer>