"Ojama shimasu" is an extremely common expression in Japanese, most typically said when entering someone else's home as a guest.
The meaning of the phrase is something along the lines of "Please pardon the intrusion." It is made up of three distinct parts:
1. O - an honorific prefix indicating respect for the person you are speaking to
2. jama - a noun meaning "interference"
3. shimasu - the polite present/future tense of the verb "to do"
So literally, all you are saying is "I am interfering" in an extremely polite way. So far, not all that interesting.
But there are two reasons why this phrase is so fun. First of all, the sound makes it pretty fun to say. The "jama" in "Ojama" (rhymes with "Obama") reminds me of English referents like "pajama," "Jamaica," and "jammin'", all of which conjures up some strange but fun imagery.
Second, if you know Chinese characters, you will definitely do a double-take the first time you look at the kanji characters for "jama." Although the Japanese word "jama" literally means "interference," the two Chinese characters for "jama" (邪魔) literally mean "evil magic." And this is not just one possible reading among many - nope, the two characters only and unmistakably mean "evil magic."
Which means when you say "Ojama shimasu" upon entering someone's home, you are also saying "I am about to (honorifically) do some evil magic."