The DOM is only applied on new SFP (SFP-GE-S/L) and not on old SFP(GLC-SX-MM/GLC-LH-SM). Below are the description of DOM found from wiki.
Digital diagnostics monitoring
Modern optical SFP transceivers support digital diagnostics monitoring (DDM) functions according to the industry-standard SFF-8472. This feature is also known as digital optical monitoring (DOM). This feature gives the end user the ability to monitor real-time parameters of the SFP, such as optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage.