Robert Oster has two lines of ink: Signature and inkArt.ink. The former is readily available. The latter doesn’t appear to be available currently. I used the Signature ink for my comparison, but, for giggles, I included a chromatography strip of the inkArt.ink (IAI).
For those familiar with Organics Studio, you’ll know that at any given time, a certain ink may or may not be readily available. For that reason, I’m dubbing it “usually available.”
The Inks
Price: ~$14 USD
Bottle Size: 55ml
Price Per Ml: $0.25
Availability: Usually available
Robert Oster Morning Mist
Price: ~$17 USD
Bottle Size: 50ml
Price Per Ml: $0.34
Availability: Readily available

If you have any suggestions for future comparisons, let me know in the comments. And check out my other Ink Dupes posts.