There were no direct Old Testament commands for baptism, except for ritual cleansings such as in Numbers 8:15. However, the Jews were familiar with the Christian concept of baptism as a matter of practice. The following is a helpful article from the Evangelical Dictionary of Biblical Theology:

BAPTISM
The Greek root-word baptizein means to plunge, immerse, sink; hence to wash; to be immersed, overwhelmed (in trouble). From Jewish rules of purification concerning ritual uncleanness the word gained a technical religious connotation implying "purification" from all that might exclude from God's presence.

When, at the diaspora, numerous Gentiles sought admission to Israel, the required public repentance and acceptance of Mosaic Law was accompanied by immersion in water, symbolizing and effecting religious, moral, and ritual cleansing from the defilements of paganism. Ancient Jewish discussions (echoed in 1 Cor 10:2) support a pre-Christian date for this proselyte baptism. This is why John's baptism needed no explanation, though his authority to perform it was challenged and his demand for purification of "children of Abraham" gave deep offense (Matt 3:7-9; John 1:19-24).
(from Evangelical Dictionary of Biblical Theology. Copyright © 1996 by Baker Books. All rights reserved. Used by permission.)