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 an 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.
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.