Thomas comes off as a smooth-talking, streetwise cat who relishes his life of freedom out in the open with no rules and no responsibilities, all of this changes when he meets Duchess for the first time. He falls in love with Duchess when he first meets her, and he quickly starts flirting with and serenading her - although he was obviously a little turned off upon learning that Duchess had children.
However, Thomas almost instantly bonds with Duchess's kittens (who return the sentiment), and he becomes like a father to them. Also, when faced with adversity, Thomas will indeed rise to the occasion to protect the ones that he cares for and loves.
In the film, O'Malley appears singing a song about himself after Edgar kidnaps and tries to get rid of the cats, but is chased off by a couple of dogs. He is instantly attracted to Duchess because of her appearance but is slightly put off when he learns she has children, despite bonding quickly with them. However, he soon has a change of heart and helps Duchess and her children get back to Paris safely. During their return to Paris, he becomes smitten with Duchess once again and starts being fond of her children as well as her. In the big fight scene, he, Scat Cat, and the Alley Cats battle Edgar as he tries to send Duchess and her kittens to Timbuktu. He saves Duchess and the kids and becomes a part of the family.
Thomas O'Malley, along with his alley cat friends, made several appearances on Disney's 2001-2003 television series House of Mouse. As a running gag, he and Scat Cat's group were often booked to sing at the club, but an incident of some sort would always result in them being fired (such as Donald booking the Big Bad Wolf to sing instead) or forced to leave (such as being chased by Pluto). In the episode "Max's Embarrassing Date", O'Malley and Duchess are seen next to a tipped table covered in milk after Goofy has an accident.
Like Tramp in Lady and the Tramp, O'Malley is street-smart. The contrast is Tramp married Lady and had puppies (including Scamp) of his own while O'Malley married Duchess and became a stepfather to her three kittens.
Eventually, O'Malley appears as a silhouette in the back railing of a passing train car right before on in the 1988 film Who Framed Roger Rabbitbefore all the Toons run out of Toontown and into the real world to have a look at Judge Doom's dead body.
Although O'Malley does not appear in The Fox and the Hound 2, Zelda, the cat friend of Dixie, briefly mentions that she has been the girlfriend of "a romeo named Thomas O'…" but before she had time to finish its sentence, Dixie cuts the word to her.
In the Italian dub, O'Malley is referred to as "Romeo". He is Roman and his most famous sentence is "'Io so' Romeo, er mejo der Colosseo" ("I am Romeo, the best of the Colosseum").
In the French dub, his full name is Walter Giuseppe Désiré Thomas O'Malley.
Between the 1970s and early 1990s, Thomas O'Malley served as the sole representative of the film alongside Scat Cat and the Alley Cats. Both eventually got replaced by Marie as the main representation of the film starting in the mid-1990s.
Duchess jokes that his name "cover[s] all of Europe". This is true for most of his name: DeLacey and Casey are Gaelic names, Giuseppe is Italian in origin, and O'Malley is Irish. However Abraham is a Hebrew name and Thomas is Arabic, which are Middle Eastern languages.
Initially, O'Malley was to be drawn with stripes, to resemble a tabby cat. However, as Wolfgang Reitherman learned when animation on Shere Khan was being done for The Jungle Book, animating stripes is difficult and expensive. Had the sequel gone forward, he would have received the stripes he never got in the original film.
Thomas only mentions Marie's name once, but has never once mentioned Berlioz's name, and has never once called Toulouse by his real name.
Thomas is the first stepfather featured in a canonical animated Disney film.
Thomas is the only cat that's never called Mr. O'Malley by Berlioz.
Thomas is the only cat not to wear a ribbon until the end.