Custom octets ?

Hi
I'm a newbie about opmanager.

I want to monitor a custom field like Interface octets. this field is defined by counter32 type and the value is increasing continuously.

I defined a new monitor using expression.

.1.3.6.1.4.1.2021.255.1.3/$DELTA_TIME

but its result only represent (CURRENT_VALUE / $DELTA_TIME).

How can I get delta about OID values?