The letter p "consumption" seems odd for me. Why does it not similar to "collect"+"ion"->"collection"? Is there any other similar noun?
Both consumption and collection are derived from Latin past participle stems, consumpt-, collect-. The -tion suffix is an extension of the PaPpl -txx stem.
The difference in the two lies not in the -tion forms but in the underlying verbs. Our verb consume is derived from the indicative consumo rather than the PaPpl consumpt-, whereas our verb collect is derived from the past participle collect- rather than from the indicative colligo.
The Latin PaPpl is most often constructed by adding -t- to the stem, which in many contexts changes the final sound in the stem. For instance:
adding voiceless /t/ to a stem ending in voiced /g/ may result in /kt/ (spelled ‹ct›), with the /g/ "devoiced" to /k/. Compare correct and corrigible.
adding /t/ to a stem ending in voiced /m/ may cause the sound to be "closed" with the voiceless stop /p/ pronounced at the same point, anticipating the voiceless stop /t/. Compare redeem and redemption.
In Oald's pronunciation the p is pronounced. http://www.oxfordlearnersdictionaries.com/definition/english/consumption?q=consumption
I have no sources and I'm horribly unqualified, but... to me this is reminiscent of the German "Fugenelement," which is a sound that is added based on phonetic context. In other words, in this example the "p" kind of rolls off the tongue anyway, so it is added to transition from the bilabial nasal [m] to the denti-alveolar stop [t]. A similar phenomenon: have you ever heard people pronounce "hamster" as "hampster"? Again, don't quote me on anything; this is just food for thought.