The trailer featured scenes from the programme while voiceover said: "Welcome to the OC, bitch."
A total of 13 viewers complained that the promo, broadcast between 6pm and 7pm, was inappropriate because many children were watching at the time.
Ofcom ruled that because people do not specifically choose to watch a trailer, many viewers would not understand the context for the use of offensive language as featured in 'The OC'.
Ofcom found the trailer in breach of the Programme Code for Family Viewing and Trailers and Programme Promotions.
Separately, the media watchdog investigated Sir Elton John after he swore on-air repeatedly during Chris Moyles' Radio One breakfast show last November.
The incident began when Sir Elton used the word "fucking", when describing how hard it was to get out of bed to appear on the early morning programme. He believed he was off air at the time.
When he realised his slip-up, the singer apologised but later in the interview went on to say "bollocks" and "bugger", teasing Moyles that he had an urge to say rude words as 10am approached.
Sir Elton finished off by asking Moyles whether it was acceptable to use the work "wank".
Ofcom found that the use of the first swearword was a genuine mistake and the further use of mild swearwords was only intended to be mischievous and light-hearted.
In view of the nature of the error, the on-air apologies and the caution taken, the matter was resolved.