I am pretty sure I understand what you want to do, so based on my understanding here is what I think you should do:
Go to "Attributes" on the WooCommerce sidebar in your admin area. Type in "Quantity" then click the button to "Add new attribute." Now the Quantity attribute should appear on the screen with a button to "Configure Terms." Click Configure Terms.
Now type "1/4 oz" in the Name field then click "Add new term" (down towards the bottom). Repeat this step for your remaining quantities.
Now go to the product editing page for the Body Oil. In the area for "Product Data" change the product type to "Variable Product." Now click on the "attributes" tab. Select "Quantity" from the dropdown menu then click "Add." Now you need to save these new changes.
Once you save that draft, scroll back down to your Product Data section and click on the tab for "variations." Look for the button that says "Link all variations" and click it. The page should then populate with a list of variations. I think in your case there will be three of them. Click on each one individually and provide the price info and perhaps the quantity in your inventory. The product can inherit the other details like tax, shipping, etc. from the parent product (it pulls the info from the other tabs in the product data area). Now you should update your page and view the product. I think that will accomplish what you are trying to do.
Let me know!