Every stock token in your wallet is a card. Cards are what you borrow against, and paying with oak adds small new ones.
Your cards
$4,812.40
connect to see yours
Your cards

TSLA
10 shares · locked in a loan
$3,673.00

NVDA
6 shares · free to borrow against
$1,305.06

COST
0.031 shares · cashback
$28.40
Cashback you earned by paying
Pay Costco with oak and 1% of the bill comes back as COST shares, a new card here.
Cashback when the merchant has no stock

NVDA
your default reward stock
A coffee shop is not on the stock market. When you pay a merchant like that, your cashback lands in this stock instead.