What specific OEM graphics card are you looking at? I have installed a Asus RTX 2080 2.7 slot card that is almost 12 inches in length. This card is installed in a mid-tower chassis. The Obelisk is a uATX chassis.
I have not worked with this chassis. I can tell you HP ships this chassis with RTX 2080 graphics cards. It looks like HP uses about a 10.5 inch modified GTX or RTX graphics card. The card length and TDP is modified to fit in this chassis and work with some lower powered HP factory installed power supplies.
Some retail GTX and RTX cards (like the Asus card) are dual slot at the PCIe backplate but can occupy up to 2.7 slots in the motherboard.
You would have to measure to see if you can go with a longer card that takes up more than 2 slots across the motherboard.
The 2080 requires a minimum 650 watt PSU. You may have to upgrade the PSU if you buy a retail graphics card that fits in your chassis.