Today, the Half-Life: Opposing Force CD key is a relic. When Steam launched in 2003 and later absorbed the Half-Life catalog, Valve offered a "CD key redemption" process. Players who still had their original 1999 sticker—or had written it on the disc—could type it into Steam and receive a permanent digital license. But many keys were lost to scratched discs, thrown-away jewel cases, or the simple decay of paper.
If you have a physical CD key from an old box (like the Platinum Collection), you can often redeem it to receive the entire (including Blue Shift, Team Fortress Classic, etc.). Launch the Steam client and log in. Click Games in the top menu bar. half life opposing force cd key