A Cash ISA is effectively the same as a regular savings account, except returns aren’t subject to income tax. As such, it is risk-free, although your returns may not necessarily keep pace with inflation.
With a Stocks and Shares ISA, however, your money is invested in equities which are traded on the stock market, or collective investment plans which themselves invest in equities. The price of your investment can go up or down, so you may get back less than you paid in. OakNorth currently only provides Cash ISAs.