You treat the oil as if it is a solid material. You can work out the weight either from the volume you take if you know its density or simply weigh a small amount in a glass vial. The assay is simple as you may dissolve the oil in methanol, hopefully your oil is miscible with methanol. DPPH is soluble in methanol too so you can do the assay in microtiter plate using ELISA reader or any kind of spectrophotometer that detect in the purple-blue colour range:

Dissolve you DPPH in methanol - I use 0.1 mM solution

Dissolve your oil in methanol – Say 5 mg/ml and serially dilute it to get a concentration gradient. A 2-fold dilution or 3-fold dilution is good.

Add 100 microliter of your drug concertation into each well of a microplate.

Add 100 microliter of your DPPH solution – final volume 200 microliter per well.

You set up control, positive control and blank (no drug no DPPH) in these wells – Positive control could be known antioxidants.

Incubate for 20 minutes at room temperature

Read absorbance at 550 nm or in that range.

You can scale up if you are using a different volume or vials in the absence of microtiter plates.

You can use anyone of our publications in this area but the assay is as simple as this.

Thank you for your guidance.

May be helpful

Article DPPH assay of vegetable oils and model antioxidants in proti...

Thank you

Hello, This may be helpful as I recently did it in this article:

https://www.researchgate.net/publication/355197776_Efficiency_of_sea_buckthorn_extract_in_oxidative_stability_improvement_of_high_oleic_sunflower_oil