• The osz package is actually a zip file, and will compress your beatmap, although not very much, since audio, video, and images are usually compressed very well. Zip and other archives are all lossless compression, otherwise you wouldn't be able to get your data back.

• Videos are usually where people go wrong. There are thousands of video codecs out there, and believe it or not, youtube is usually a terrible place to get videos. For the best quality, you should always downsize from a high resolution source (720p youtube is fine for this).
  
  h.264 and XviD are very good codecs to use, but like most video codecs, they are lossy, so do not re-encode multiple times, but encode from the high resolution source.
  
  Recommended reading: How to edit/re-encode video

• Now that there is a limit on audio bitrate, it's not as much of a problem anymore. However long songs at 192 kbps will undoubtedly take up most of your space. I recommend using a variable bitrate between rate factor 2 and 5 (I try to stick around 4). With a variable bitrate, you get more compression in low activity areas, and more quality in higher areas.
  
  
  
  Programs such as Audacity 1.3.13 do an excellent job of saving variable bitrate audio with the correct plugins.
  
  Osu accepts 2 major file formats: mp3 and ogg vorbis. Vorbis has a clear advantage over mp3 at low bitrates (under 128 kbps), but suffers from lack of support by proprietary software like apple, since mp3 is the standard for audio. However, there are severely limiting returns on using anything higher than 128 kbps. For osu! it doesn't really matter which one you use.

• 
  There are 2 major image formats which make up most of the images you find online: jpg and png. Png uses lossless compression which makes it ideal for small and or simple images. Complex images are better suited for jpg. The majority of image editing programs however do not naturally use png and jpg compression to the most of their ability however, leaving you with 4 kb pngs of 1 color, which could be compressed up to 400 times smaller.
  
  Gimp actually does a good job with jpgs.
  
  
  
  There are four important things to check: Optimize, Progressive, subsampling, and quality.
  Optimize is just a way to compress the jpg without losing quality and should always be checked.
  Progressive is a method to make jpgs "fade in" when downloaded from the internet. It has a side effect of making the image smaller so long as the image is over 20 kB.
  Subsampling is a way to improve quality or compression. No subsampling (1x1) will make text and lines much clearer at the cost of space. 2x2 subsampling does very well for photographs which do not have distinct lines (like text).
  Quality is the easiest way to keep the image looking good. I use 95, since the gains after 95 aren't really worthwhile.
  
  Pngs are actually much simpler to compress, but GIMP does not do it very well. Optipng and Pngout are two of the best tools to compress pngs. Optipng can find the optimum settings for the png, and pngout re-compresses the image using kzip. Often times just using pngout will reduce the image size by 20%.
  
  
  For even better compression, you can use a script which combines multiple compressors and brute forces close to the smallest file possible. The script's name is pngslim.
  

some results

Derekku wrote:

After a discussion with peppy, we suggest that VBR should be avoided if possible. However, a lower quality around 140-160 kbps VBR (like you mentioned) should be fine.

Equivalent wrote:

I think it not the offset, but like V2 VBR sometime the mp3 will go beyond he limit of kbps rules which like it goes 198kbps to 200kbps in certain part. Mostly the issue here I think is V2 VBR. Where the bitrate is like doubtful to be unrankable or rankable within the requirement range.

Offset in VBR maybe another issues as I experienced someone saying my map is having late offset while at my side it perfectly good.