* January OSP down $1.85 a barrel from previous month * Cut in line with reduction made by Saudi Arabia (Adds detail, background) SINGAPORE, Dec 11 (Reuters) - Kuwait set the official selling price (OSP) for its crude oil sales to Asian buyers for January at $3.95 a barrel below the average of Oman/Dubai quotes, a trader said on Thursday. The January OSP, down $1.85 a barrel from the previous month, is Kuwait's lowest since December 2008, Reuters data showed. The cut was in line with a reduction made by Saudi Arabia for its Arab Medium grade last week. ...