See: Case-Insensitive RedirectMatch

# Redirect First
<IfModule mod_rewrite.c>
RedirectMatch 301 (?i)/post-slug/?$ https://othersite.com/some_url/
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

See this comment for further explanation.

Thanks for your answer, its works !
I had already try to put my rules before the WordPress Permalink Rules, but I was using RewriteRule instead of RedirectMatch.

Thanks again, have a great day !

So I had to write the custom URLs in another .htacess file, and put it in the parent folder (the same as /www). I’m not sure it’s a really good thing to do, but it works.