Agates are found along many beaches on the Oregon Coast that are adjacent to cliffs and streams. In addition to finding a good location for agates, beach conditions are a critical factor. Typically, sand builds up on beaches during the summer and covers gravel beds containing agates, while winter storms loosen agates from cliff sides and strip the sand off beaches to reveal these deposits. This usually makes November to March the best time to discover significant agate deposits. Your best best in October would be to concentrate on cliff-side beaches at stream outfalls that wash sand away from rocky deposits. Closest to Portland, the beaches south of Cannon Beach would be a good bet. Consider Hug Point (near the waterfall), Arcadia Beach near stream outfalls or Short Sand Beach in Oswald West State Park where Necarney Creek flows into the ocean. One of my favorite beaches for agate hunting on the North Oregon Coast is the beach at Oceanside on the north side of Maxwell Mountain that is accessed by a tunnel at low tide, though it can be hit or miss depending on sand deposits.
This stretch of Coast, south of Cannon Beach and following the Three Capes Scenic Route (Cape Meares, Cape Lookout and Cape Kiwanda) makes a nice touring route for spending just a few days at the Coast. You can make this trip a loop by driving to the Coast via US 26W to Cannon Beach, south on US 101, following the Three Capes Route from Tillamook, then returning to Portland via OR 6E from Tillamook. You should note that the road north of Cape Meares is indefinitely closed due to slides, so you have to reach Cape Meares and Oceanside by driving through Netarts.
Happy agate hunting!