It depends upon the dosage and pattern of usage. For most adults the so called "Blocking Dose Threshold" is 70 millgrams of methadone. If you are only a casual user of it you can still often gain some benefit from other opiates/opioids, like your oxy. The "Blocking" effect is really dependent upon regularly patterned usage and even then it takes until at least the 4th week to reach. That said, combining opiates/opioids is not a good idea ESPECIALLY with methadone.
Substances are subject to a value known as Metabolic Half Life (as opposed to Radioactive Half Lfe). MHL, usually discussed simply as "Half Life" represents how long it takes the average adult to fully metabolise a substance (as in how long a substance "lasts"). Methadone's Half Life is thirty-six hours. Its psychoactivity, or "high", lasts up to eight hours. So, you take a dose of methadone, several hours later you are sober. You feel like getting high but your body still has dozens of hours to go before the methadone is flushed from your body. This is why methadone is a terrible drug for recreational use, and even worse for pain. People rarely understand the mechanics and amazingly this includes physicians who prescribe it for pain.