API Calling By Oracle PL-SQL/APEX Using UTL_HTTP/ APEX_WEB_SERVICE and ACL Configuration

API Calling Using URL_HTTP

declare

    v_req       utl_http.req;
    v_res       utl_http.resp;
    v_buffer    varchar2(4000); 
    v_body      varchar2(4000) := '{"field":"value"}'; -- Your JSON
begin
    -- Set connection.
 v_req := utl_http.begin_request('http://yourapi_base_url/operation','POST');
    utl_http.set_authentication(v_req, 'username','password');
    utl_http.set_header(v_req, 'content-type', 'application/json'); 
    utl_http.set_header(v_req, 'Content-Length', length(v_body));
    
    -- Invoke REST API.
    utl_http.write_text(v_req, v_body);
  
    -- Get response.
    v_res := utl_http.get_response(v_req);
    begin
        loop
            utl_http.read_line(v_res, v_buffer);
            -- Do something with buffer.
            dbms_output.put_line(v_buffer);
        end loop;
        utl_http.end_response(v_res);
    exception
        when utl_http.end_of_body then
            utl_http.end_response(v_res);
    end;
end;

But if you have Oracle APEX installed, then you can try APEX_WEB_SERVICE package (much simpler).

declare
    v_response      clob;
    v_buffer        varchar2(32767);
    v_buffer_size   number := 32000;
    v_offset        number := 1;
begin
    -- Set connection and invoke REST API.
    v_response := apex_web_service.make_rest_request(
        p_url           => 'http://yourapi_base_url/operation',
        p_http_method   => 'POST',
        p_username      => 'username',
        p_password      => 'password',
        p_body          => '{"field":"value"}' -- Your JSON.
    );
    
    -- Get response.
    begin
        loop
            dbms_lob.read(v_response, v_buffer_size, v_offset, v_buffer);
            -- Do something with buffer.
            DBMS_OUTPUT.PUT_LINE(v_buffer);
            v_offset := v_offset + v_buffer_size;
        end loop;
    exception
        when no_data_found then
            null;
    end;
end;

If you get an ACL exception, then you have to create ACL to open TCP port to connect with REST API.

BEGIN
    DBMS_NETWORK_ACL_ADMIN.create_acl (
        acl          => 'acl.xml',
        description  => 'Connecting with REST API',
        principal    => 'YOUR_DATABASE_SCHEMA',
        is_grant     => TRUE, 
        privilege    => 'connect',
        start_date   => SYSTIMESTAMP,
        end_date     => NULL
    );
    
    DBMS_NETWORK_ACL_ADMIN.assign_acl (
        acl         => 'acl.xml',
        host        => 'localhost', -- Or hostname of REST API server (e.g. "example.com").
        lower_port  => 80, -- For HTTPS put 443.
        upper_port  => NULL
    );
    
    COMMIT;
end; 

I assume your REST API is protected by basic authentication scheme (user and password). To keep this example simple i used HTTP. If you have to connect via HTTPS, then you have to change TCP port in ACL and configure Oracle Wallet for your Oracle database instance.