If you upload a JPEG, Flickr does not modify the Original-size image in any way (apart from changing the filename), at least not on a Pro account.
I tested it out by uploading a full-size, 100% quality JPEG to Flickr then re-downloading the Original size image and comparing it with the original (using a comparison tool called Beyond Compare). The two files are identical, byte for byte. That means not only has the image not been compressed but all the original metadata (Exif etc.) is also intact.
I'm on a Pro account so I can't be sure about Basic accounts but my guess is that the same applies, they just restrict you from viewing it.
I also tested a TIFF and the Original image on Flickr is a JPEG. So in the case of TIFFs, they are compressed. (All JPEGs are compressed so some extent, even if you choose 100% quality - hence the size difference between JPEG and an equivalent TIFF.)