Is the seperation of data from different applications via Oracle schemas considered secure?

I am evaluation the pros and cons of having different oracle schemas vs. seperated oracle servers. Having a dedicated server for each application is really expensive and I only want to consider this, if the security benefits are worth it.

One of the main sticking points is the topic of SQL injection. In case application A has an SQL injection which compromises all the date in schema A in the oracle database. Would it be possible for an attacker to access the data in schema B (in the same oracle database) as well?

I am leaning towards the opinion that seperation via schemas is enough except for the most critical of data stores. Is this best practice and what are the main factors to consider?