It depends on the symptom. A small oil seep that leaves a coin-sized spot, a faint tick that goes away after a minute of warm-up, or a steady check engine light with the car running normally can usually wait a few days for an appointment. Keep an eye on the oil level in the meantime. A deep knock, a flashing check engine light, a red oil pressure warning, a temperature gauge heading for red, or thick smoke from the tailpipe is a different story. Pull over when it's safe, shut the engine off and get help.
Engines punish waiting more than most parts of a car. An oil leak left alone can drip onto the hot exhaust or run the level low, a misfire can overheat and ruin the catalytic converter, and a knock usually means a bearing wears a little more with every mile. What could have been a gasket or a coil turns into a rebuild or a replacement engine. When in doubt, don't push it: have the car towed to the C&C shop you choose, or drive it in gently while the problem is still small, and we'll tell you where it stands.