Is it legal to use someone else's credit card with their permission? What about forging a signature, also with permission?
It is legal for you to use another person's card with their permission, since it is not statutorily forbidden and you have no contractual obligation to the card company. It may be contractually forbidden (by the card company) to let your card be used in that way, though I've never heard of any such prohibition. It is not forgery, under ORS 165.007: there is no intent to harm. There is also credit card specific ORC 165.055, which also does not apply because use of the card is with permission of the issuee.