It depends on the exact versions:
- Using 11g Client with 10g Database says
"Metalink 207303.1 is the client/server interoperability matrix. If by 11g you mean 11.2 and by 10g you mean 10.1, the 11.2 client will not work with a 10.1 database. If the client is 11.2 and the database is 10.2, it should be at least 10.2.0.2. If the client is 11.1, the database should be 10.1.0.5 or 10.2.0.2 or higher."
- Is it ok to use oracle 11g client with a 10g server? agrees with that (with less detail)