Hello!
1. How to move folders:
\oc-content\uploads\0\
\oc-content\uploads\temp\
to the root of the site (for example in "domains" folder)?
2. What is the folder "temp" for? How often should it be cleaned?

Osclass does not support change location of those folders.
Temp one should be cleaned by each feature that use it

1. I see that the 'Temp' folder in \oc-content\uploads\ is not cleared. Is it problem with osclass v8.0.2 settings?
2. What is the table 't_log' in database for?

can we make white label for change directory folder name like oc-content, oc-admin and favicon just one click from admin panel in next version ?
now when we access image URl it will show original osclass favicon also when acess admin will show osclass favicon too,

1. I see that the 'Temp' folder in \oc-content\uploads\ is not cleared. Is it problem with osclass v8.0.2 settings?
2. What is the table 't_log' in database for?

It should be cleaned by CRON once per hour, but it cares just about Osclass data (QQ files).
    $qqprefixes = array('qqfile_*', 'auto_qqfile_*');
    foreach ($qqprefixes as $qqprefix) {
      $qqfiles = glob(osc_content_path().'uploads/temp/'.$qqprefix);
      if(is_array($qqfiles)) {
        foreach($qqfiles as $qqfile) {
          if((time()-filemtime($qqfile))>(2*3600)) {
            @unlink($qqfile);
          }
        }
      }
    }