If a corporate action such as a merger, acquisition or spin-off occur before the expiration of your contract, the clearinghouse will typically announce an adjustment to the amount or name of the security deliverable under the contract. When such adjustments occur, the party delivering the underlying (i.e., short holder of a call or long holder of a put) is obligated to deliver the adjusted security at the option strike price.
For example, if hypothetical company ABC approves a takeover of its shares by company DEF at rate of 2 shares of DEF for every 1 share of ABC, then the holder of a long ABC call would receive 200 shares of DEF instead of the standard 100 shares of ABC upon exercise. If the takeover resulted in fractional shares, the clearinghouse would typically adjust that fractional quantity to provide for an equivalent value to be delivered in cash.
In the case of U.S. exchange listed securities options cleared by the OCC, the rules of the clearinghouse provides for an adjustment panel which consists of representatives of the listing options exchanges and one OCC representative (who only votes in case of a tie) and this panel determines how to adjust based upon general adjustment rules. OCC publishes Information Bulletins outlining the terms of adjustments on its website.