Hi Simon,
At the bottom of the login page there's a link to recover the password: "Lost your password?".
However, if there's no link, you can update your admin user with this sql sentence:
UPDATE osc_t_admin SET `s_password` = sha1('your password') WHERE `pk_i_id` = 1;
** NOTE: Maybe your table has another name, for see all your tables write in the command line: "SHOW TABLES;"
Regards,