Usually yes, provided a full continuous year of qualifying Ontario employment can be located within the three-year look-back. American L time is not credited toward the year, but it does not break the continuity of the Canadian employment, and the same nine months are charged against the seven-year L-1A maximum.
Two separate calculations, one timeline
Draw the last four years as a line and mark the Ohio assignment on it. The first calculation asks whether one unbroken year of employment with the qualifying organization abroad can be found in the three years preceding the filing. Because the American interval is not counted as part of that year, the useful Ontario months are those either side of it, and they must include twelve consecutive ones.
For Dermot, whose Ontario service resumed immediately on return and has continued since, that year is easy to identify and easy to evidence with payroll records and a letter setting out unbroken dates. The second calculation is unrelated to eligibility and concerns how long he may stay. Time previously held in either L category counts against the seven years available to an L-1A transferee, so his effective remaining runway is closer to six years and three months.
Anyone planning a permanent-residence step later should count from that shorter number.