list of file you have to modify

config-inc.php
install.php
database-api.php
custom_field-api.php

****config-inc.php

keep $g_hostname blank

$g_hostname='';
$g_db_username='username';
$g_db_password='password';
$g_db_database_name='database name';
$g_db_type='oci8;

****install.php

add in line 211 in switch($f_db_type)

case 'oci8:
$t_support=function_exists('OCILogon');
break;

add in the driver list this line

<option value="oci8">Oracle(oci8)</option>

****custom_field-api.php

replace the query in line 629 with

 $query = "SELECT cft.id id, cft.name name
            FROM $t_custom_field_table cft, $t_user_table ut, $t_project_table pt, $t_custom_field_project_table cfpt
                LEFT JOIN $t_project_user_list_table pult 
                    on cfpt.project_id = pult.project_id and pult.user_id = $t_user_id
            WHERE cft.id = cfpt.field_id AND cfpt.project_id = pt.id AND ut.id = $t_user_id AND 
                ( pt.view_state = $t_pub OR 
                ( pt.view_state = $t_priv and pult.user_id = $t_user_id ) OR 
                ( pult.user_id is null and ut.access_level $t_access_clause ) )
            GROUP BY cft.id, cft.name
            ORDER BY cft.name ASC";

**database-api.php

add in function db_helper_compare_days

case 'oci8' : return "(($p_date1 - $p_date2)".$p_limitstring.")";

Really, with all issues, mantis run with database oracle