O'Neill will be with the London club until 3rd May and joins Burnley team mate Alex Cisak who has already made four appearances for them since going there on loan, keeping his first clean sheet last Saturday against former club Oldham.
Loans away from Turf Moor have been commonplace for O'Neill, who signed a new two year deal with Burnley last summer. Last season he was with York from October until the beginning of January before moving to Southend later in the month, that deal being cut short because of an injury sustained on his debut.
This season he's already been at Scunthorpe for whom he made 15 appearances and now he'll spend the remainder of this season at Orient.
He goes straight into Fabio Liverani's squad for their away game against Walsall this Saturday.
O'Neill was originally signed by the Clarets from Mansfield in the summer of 2012 but his only league appearance to date came in the last match of the 2012/13 season against Ipswich when he replaced Kieran Trippier in stoppage time.