Contracts are fully-funded. When you buy an option, you will pay the seller the $ premium at time of trade. You need dollars in your account to purchase options. See Fund Your Account.
Buying a call: you are paying for the option to buy bitcoin at the strike price on the expiration date. See Call Options for more information on trading calls on Omni.
Buying a put: you are paying for the option to sell bitcoin at the strike price on the expiration date. See Put Options for more information on trading puts on Omni.