Quoting: drambui
they can trade him 6 months after.
They can not trade him for 1 year after he signs an OS, the 6 months (actually it is Jan 1st) deadline applies to negotiating an extension.
What are the details for the club receiving the offer sheet?
The player must be an RFA and therefore the receiving club must have made a qualifying offer (QO) to the player by the later of:
5:00pm ET on June 25th
5:00pm ET on Monday following the NHL Entry Draft
If the player has not received a QO they will instead become a UFA, for further details please see the Qualifying Offer Calculator
The club that receives an Offer Sheet has 7 days to match the offer or accept the draft pick compensation:
If the receiving club accepts the Offer Sheet, they receive the compensation package which coincides with the annual average value of the offer made from the submitting club
If the receiving club matches the Offer Sheet, they are bound to the contract details outlined in the offer, and must respect all aspects of the contract
If the receiving club matches the Offer Sheet, they cannot trade that player for 1 year from the date of first refusal