Greetings @Sarafeena_Sage!
Congrats on your beautiful harvest of Calendula- they are practically glowing with all of your love and care! I know they will make a very potent healing oil for you 
The parts of the plant to use (or not use) often depend on a few factors:
- the recipe
- the method of extraction
- the purpose of the oil
In this case where the recipe doesn’t specify, I’d consider what type of oil you are making and how. If it’s a type of oil to be used on food, know that the green stems of many plants can give a bitter taste and even overpower the taste of the petals.
If the oil is to be used as a skincare product, however, the taste doesn’t matter much at all 
I agree with Francisco here- feel free to leave the green base of the plant/stem on while you craft the oil, but I’d personally suggest to cut it as close as possible.
While the inclusion of some green parts won’t detract from your oil, when it comes to Calendula, the medicinal properties you are seeking are mostly strongly found in the yellow blossoms- not the green stems.
Wishing you a blessed calendula harvest and a fun time crafting your oil- let us know how it goes! 
