The one that was seen as certain already in 2010, so much so that the MOD booked in its accounts 500 million pounds well before the actual deal was signed.
Back then, negotiations verted on two options, each about 24 Typhoon jets. One option for Oman was to buy Tranche 1s modernized (Block 8, FRG4 standard, in other words) from the RAF, which would have further slowed down the buildup of the Typhoon force but would have greatly helped the MOD balance the books. The other option, the favorite one, was for Oman buying 24 slots from the Tranche 3B allocations of the UK, allocations that are not going to be taken up by Great Britain.
However, the order failed to materialize, and last month Oman ordered a further package of 12 (2 twin and 10 single seats) F16s fighters from the US, in a 600 million dollars deal. BAE systems had confirmed, literally weeks before, that negotiations for the Typhoon buy were ongoing and that they were absolutely certain that an order would come.
My interpretation of the day was that, for financial and political reason, Oman had probably decided to split the 24-jets buy in half.
There is now indications of it being the exact interpretation. The government of Oman has requested that BAE Systems submit a formal bid for the supply of Typhoon fighter aircraft, for equipping one Squadron (so roughly 12 airplanes), along with training and support. [the training voice is going to be interesting: how will they sustain a single-squadron fleet? With just 12 airplanes, no OCU and no margins, they are going to have a very small number of Typhoons ready for actual use in fighting.... Will their pilots come to the UK for training and conversion on the Typhoon?]
Anyway, times are going to be, once more, long. BAE expects negotiations to be over by the end of the year (!) with deliveries of the first jet coming 36 months later.
Still a good news for the Typhoon programme and for british industry and jobs, and perhaps it is a good news for the MOD, too, to a degree.
However times are long even in the optimistic plans (last time BAE talked about this order, in early 2011, the deal was to be signed "within months") and the order is halved compared to expectations.