From what I've read (mostly on here) it sounds like Safe-T-Sorb holds up better long term then oil-dri.
Both just serve to help transfer nutrients from the water column to the plants roots (the higher CEC rating).
Do you have anything else currently in your substrate, or is it strictly gravel?
the OD/STS can be used instead of gravel, so you could just mix it in, or dump it on top, or whatever. It would probably be better if you had some nutrient source in your substrate, either a dirt layer under the gravel/OD/STS, or root-tabs or something underneath.