The way oxwall is designed around that rewrite rule, all you need to do is to tell the htaccess to accept your subdomains. 

so leave the code for oxwall in there

but right above the part that you show in this post put this

 #added to access custom files - sub domains or special folders
RewriteCond %{HTTP_HOST} !blablabla.org$           #use this for subdomains
RewriteCond %{REQUEST_URI} !/customfolders/       #use this for custom folders
RewriteCond %{REQUEST_URI} !/images/                 #same for folders
RewriteCond %{REQUEST_URI} !/googlewhatever701ad.html   #use for custom file names
#end of add

So you basically have to designate everything that is not a plugin, theme, or that came standard with the script download.  Just list it per the examples and you will be fine..