Hi,
This question is going to take alot of concentration to understand so I apologise in advance!

For one of my website I use OScommerce; and this exists in the subfolder /catalog.
I have installed a redirected using cPanel; this redirects visitors who enter at the base mysite.com/ to mysite.com/catalog.
My question is
a) for search engine friendliness would I be better using an HTML redirect i.e. have a mysite.com/index.html page that forwards the user to mysite.com/catalog OR is the server redirect fine
b) If neither of these are ideal, does anyone know if there is a quick way of getting a mysite.com/index.php to display the relevant homepage (i.e. currently .com/catalog/index.php) OR is there a way of simply pushing the contents of /catalog onto /public_html/ folder?

Thank you for any help whatsoever.

Setting up the redirect through cPanel is fine. Just make sure you set it for a permanent redirect. (I think temporary is the default). If you have it set for temporary delete it in cPanel and reset it again as permanent. A permanent redirect is the same thing as a 301 redirect which is the only safe way as far as seaarch engine friendliness is concerned.

Just out of curioisity if you want people entering the site to go directly to the store why not have installed the store in the root instead of the subfolder?

Well I can ask the same question now - I did see the option for it right away so I assumed you couldn't...

Thanks for the advice; it's set as a permanent redirect.

You must do redirects server-side to avoid breaking the back-button on users Web browsers!

If you want people to visit your store just send them directly to the home page and don't worry about the redirect! Unless off course you have several sites you are trying to promote at the same time. Be carefull redirects tend to agravate the normal suffer which could turn them off to your site all together.