O'Connor joined Rushden last summer after leaving Mansfield
Aaron O'Connor scored a brace to secure Rushden a play-off place and leave Ebbsfleet in deep relegation trouble.
A clever flick from striker Craig Farrell found O'Connor who drilled the opener into the bottom corner.
Farrell earned a penalty on half-time after being brought down by Leon Crooks but Paul Terry failed to convert it and put the rebounded shot over the bar.
O'Connor slipped past the away defence and smacked a left-footed shot passed Cronin for his second on 69 minutes.
Struggling Ebbsfleet will be left to rue a glaring chance missed by Scott Ginty, who broke free and went one-on-one with keeper Dale Roberts but put it just wide in front of an open goal.
Other than Ginty's chance, the visitors barely threatened a strong Rushden side and now find themselves languishing in 22nd place in the league, four points from safety with two games left.
The win means Diamonds leap-frog York into fourth and clinch a play-off place with three games of the season remaining.
Rushden & Diamonds manager Justin Edinburgh: "It was a fantastic achievement from a top-class group of footballers. In the second half we were very good and we deserved our win.
"Pleased with the clean sheet, I thought we deserved that. It's another home win and three more valuable points that take us over the line.
"We go in as underdogs, we've overachieved already, and we'll look forward to it whoever we play."
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