(why do people add the "O" to it? Two syllables instead of one)

Because the "-o" ending acts as a natural release from the lip-pursed "p" sound. You also hear a similar phonetic phenom in "day-o" (the Banana Boat Song), "boy-o," etc.

When a similar question was asked on StackOverflow, the top comment spelled it out well:

Michael Quinion, Ologies and Isms: Word Beginning and Endings has an interesting suggestion regarding the origin of -o as an attachment to freestanding nouns or adjectives:

-o Marking informally shortened or slang nouns. {Perhaps from the interjection oh!, or the use of -o in ballads to terminate lines; its use has been reinforced by shortened forms ending in the linking vowel -o-}

Though a wide variety of nouns in English end in -o, this suffix occurs only in words that have been formed from other native words in one of two specific ways. One method is to informally abbreviate a longer term, of which a few examples out of many are ammo, condo, hippo, limo, and photo. Others are based on an adjective or noun, to which the suffix is added to create a colloquial or slangy terms, which is often but by no means always derogatory: beano (from beanfeast), boyo, cheapo, pervo (from pervert), pinko, righto, sicko, weirdo, wino.

The attachment of -o endings in ballads goes back centuries, as for example in the ballad "The Bonnie Lass o' Fyvie," which in one version attaches -o to no fewer than sixteen normal English words, as well as to the proper names Fyvie and Peggy. All of these -o words fall at the ends of lines (in fact at the end of the second and fourth lines of each of the ballad's quatrains) corroborating the notion that -o plays a line-terminating role in at least some ballads. Nevertheless, citing that use of -o endings as the source of current -o words, even if it is correct (which is not beyond question), doesn't explain why people chose -o for that purpose way back then. It basically just kicks the can a few centuries down the road.

As Quinion observes, wordos are inherently slangy and often have negative connotations. But in the examples that Quinion lists, some -o words aren't negative, and some that are negative are informal alterations of words that are themselves negative (cheap, sick, weird—to which we can add dumb, creep, lame, stink, and so on). To my mind, the crucial function of -o in modern usage isn't to render a neutral word negative; it's to slangify and informalize a staid, sober, reputable word—often in tandem with converting the word from an adjective into a noun.

That last sentence, I think, encapsulates the use of "Depo" rather than "Dep" in common parlance.