Oscar left Shanghai Port for Sao Paulo last year and suffered a health emergency at the club's training centre this week.
The 34 year-old midfielder fell unconscious and was rushed to hospital where tests uncovered a heart issue.
Oscar last played for Sao Paulo in August where he suffered an injury against Corinthians. The veteran was hit by a back injury, with scans also showing a heart abnormality, reports Globo Esporte.
Sao Paulo stated this morning that Oscar "is currently in stable condition and remains under observation for further tests to clarify the diagnosis.
"As per standard procedure and respecting the player's privacy, further information will be released as soon as there is an update from the medical team, in agreement with Oscar."