Notional value is the total value your position controls. For example, you buy one $100 okex contract at $7,000 BTC/USD, which is worth about 0.0142 BTC at that price ($100/$7,000 = ~0.0142), this is your notional value of the open position of one contract. If you open ten contracts, it would be 0.142 BTC, hundred – 1.42 BTC, et cetera.
You don’t need to post the whole sum of 0.0142 BTC to open one okex contract position, leverage allows you to have just one-tenth of that or even less, but your notional value, in that case, will still be 0.0142 BTC.
Do not mistake notional value for the market value. The former is the total value of a whole position, while the latter is the price at which the position can be bought or sold.