Daniel Mandroiu had a penalty saved as Lincoln missed out on a Sky Bet League One play-off place with a 2-0 defeat to champions Portsmouth at the LNER Stadium.
The Imps started the day in sixth place but, with Oxford winning, needed three points of their own to stay there.
Late goals from Myles Peart-Harris and Paddy Lane settled the contest in Portsmouth's favour but only after Mandroiu failed to convert after referee Sunny Singh Gill ruled that he had been fouled by Tom Lowery in the area.
Lincoln created the first opening as Paudie O'Connor laid a free-kick off to Lasse Sorensen, who fired his low effort wide.
A Jack Sparkes cross fell for Colby Bishop at the other end, but he skewed his shot off target.
Lukas Jensen saved a low Bishop effort from a Lane cross and Callum Lang saw his follow-up blocked.
Lincoln should have gone ahead early in the second half, with Ben House firing over from O'Connor's knock-down before Will Norris saved Mandroiu's tame spot-kick.
Pompey went ahead in the 81st minute when Lane squared the ball for Brentford loanee Peart-Harris, who drilled in a low effort.
Lane added a second deep into stoppage-time after substitute Abu Kamara's effort came back off the post.