Merriam-Webster Online has an entry for possum. I have always learned that the proper way to spell this shortened version of opossum is to use an apostrophe, to wit: 'posssum. Has this changed? Is the presence of an entry for the unapostrophed version an indication that the spelling possum is now “acceptable?”
Never heard of using an apostrophe with this – and I’ve only been able to change from 'til to till recently and under great duress.
My OED (I think it’s the 1920s compilation) lists “possum” with no apostrophe.
Only the Irish ones.
It also attests the “aphetic” (sans apostrophe) as far back as 1613.
Clipped words used colloquially often are written without the apostrophe. You never hear of a woman going to the beauticians to get a perm’ or the truckdriver hauling a semi’. I’d assume that “possum” as colloquial for Didelphis virginiana has never been written with an apostrophe as a general rule.
However, don’t forget that in addition to the opossums of the Americas, Family Didelphidae, there are also possums. properly so called and without the leading o-, in 63 species belonging to six families of Suborder Phalangeriformes, among the Australian marsupials.
I always thought it was to be o’possum. Granted this is the first time I’ve ever written the word since about the third grade, where I distinctly remember learning that it itself was a contraction for “old possum” (for whatever reason).
M-W indicates it coming from a native word apasum. Similarly, the native word for “raccoon” (according to M-W) was arakhun, but it entered the language only in its aphetic form.
It was once the style for clipped words to use an apostrophe. However, this style has been increasingly abandoned since WWII for a more casual style, even in the most formal writing. It’s part of the same trend that takes the hyphen out of words like e-mail and on-line.
Possum became acceptable as a colloquial term a long time ago. Eric Partridge, in Usage and Abusage, allows it and doesn’t mention an apostrophe. I’d also guess that Pogo Possum may have had an influence, although it certainly would have happened, um, irregardless. O’possum as the shortening of old possum is clearly incorrect. Opossum is the formal term. There’s a 1610 reference to Apossouns, with both opossum and possum being used in that 1613 cite of Earl Snake-Hips Tucker.
As for 'til, 'till, and till, the usage note in the American Heritage Dictionary says:
I was always taught, just for the record, that 'til was incorrect and that the only acceptable form was till. Don’t believe anything about English taught to you in high school is the moral.
From Wikipedia
A possum is any of about 63 small to medium-sized arboreal marsupial species native to Australia, New Guinea and Sulawesi. The name derives from their resemblance to the opossums of the Americas and, unlike most names applied to Australian fauna in the early years of European colonisation, happens to be accurate: the opossums of America are distant relatives…Possum is also used in North America as a diminutive for the Virginia Opossum…
Thanks to all you folks for clearing that up. I had been using the apostrophe on 'possum ever since Pogo himself misled me in a Sunday strip. Another character was writing down Pogo’s name, spelling it out P-o-g-o P-o-s-s-u-m. Pogo, smirking, said, “Actually, 'Possum is spelt with an O.” :smack:
Instring. I, myself, recalled that Walt Kelly at some point used 'possum. I guess I’m behind the times by, oh, 400 years or so. 
Likewise, Australians referred to the big monitor lizards found there as *goannas, * a corruption of “iguanas,” which are found in the Americas. But this is not accurate, since monitors and iguanas are not related aside from both being lizards.
Perhaps goanna should be spelled 'goanna. 
I have seen 'possum in print, as well as 'gator. But use of the apostrophe at the beginning of a shortened word seems to be dying out. Wait! What about 'rents (parents)?
Another change I’ve noticed - it used to be standard when writing plurals of numerals or single letters to stick an apostrophe in there (“6’s and 7’s”, “p’s and q’s”). That seems to get dropped more and more lately (“the 1960s”). Good, because I never liked it - it doesn’t replace anything.