Diaconescu has offered to pay 203.149 million lei ($58.5 million/45 million euro) for the stake, the highest offer among the four received in a sealed bid auction, Remus Vulpescu said in a live broadcast of the auction by local TV channel Antena 3 (www.antena3.ro).
Diaconescu can sign the sale contract as soon as Monday, Vulpescu added.
The Romanian government had set a price of 0.1 lei per share for the 188,100,976 Oltchim shares.
Local companies Aisa Invest and Chimcomplex and German chemicals, energy and logistics group PCC SE also placed bids for the stake. Aisa Invest filed the second largest offer, of 30 million lei.
Together with the stake, Romania alo offered for sale receivables worth a combined 1.8 billion lei ($527.4 million/401 million euro) due by Oltchim to the state privatisation authority, AVAS, and state-owned power producer Electrica.
Diaconescu did not bid for the receivables, Vulpescu said.
"We will negotiate the debts to AVAS and Electrica with the state," Diaconescu said.
PCC SE and Cyprus-based Nachbar Services Limited owned 18.3% and 14.03% of Oltchim, respectively, at the end of June, data from the central depositary showed. Smaller shareholders held the balance.
(1 euro=4.5146 Romanian lei)