The group, which provided x-ray technology for Nasa's Mars Curiosity rover, said it had faced weaker demand in Japan and difficult trading in its industrial analysis business.
It also said it had taken a hit from the sudden tightening of Western trade sanctions against Russia.
While orders rose 13% to £386.6mln and revenue lifted 7.1% to £385.5mln in the year to March 31, adjusted pre-tax profit fell to £35.6mln from £47.1mln a year ago.
It also said trading in the first two months of the new financial year had been slow, although it expected a return to growth later in the year as new products gain traction.
Chief executive Jonathan Flint said the group was confident that nanotechnology would drive long-term demand for its high-tech equipment.
"This, together with the benefit of announced cost savings, mean our expectations for the current year are unchanged," he said.
Despite that, shares fell 34p or more than 3% to 1032p in early trading in London.
Analysts at Liberum Capital said the stock looked fully valued. "While Oxford Instruments remains an exciting play on scientific markets and self-help should drive the margin, residual lumpiness and tough trading conditions raise questions over the sector premium," they said in a note.