Three of the candidates - vice-president Bradley Russell, welfare officer Shonelle Eastwood and colleges and communications officer Francisco Hernandez - are OUSA executive representatives, who have been joined by student politics newcomer Logan Edgar.
OUSA is holding the by-election to replace outgoing president Harriet Geoghegan, after her unexpected resignation last month.
The candidates have run truncated campaigns because of the need to hold elections before the half-term exams, OUSA spokesman Alasdair Johnston said.
All four candidates said they had been busy trying to combat student apathy and secure votes, before tomorrow's 4pm deadline.
Online campaigning via social-networking sites was a popular strategy employed by the candidates, with visits to lectures, halls of residence, and student flats among the more traditional methods to reach student voters.
Ms Eastwood said having visible polling booths was better than expecting students to vote online, while Mr Hernandez said many students were busy with exams so it was easy to understand why some found it hard to find the time to vote.
"I've been running a grass-roots campaign and getting out to halls to set up polling booths," Mr Hernandez said.
Mr Russell said his responsibilities as interim acting president had meant finding the time to campaign had been more difficult.
His political experience as OUSA vice-president and an ability to "engage and inform" student voters would be an asset, he said.
Political novice Mr Edgar said it could sometimes be an "uphill struggle" to get students to do anything, "let alone vote".
He was confident his broad base of supporters would "front up" during elections, and pointed to the hard work and help of his campaign team.
Votes can be cast online and at polling booths set up around the university campus and halls of residence. At 5.15pm yesterday, 1546 votes had been cast online, an OUSA spokeswoman said.
