Yes, OneCare includes the core scanning components of Defender, so it disables the copy of Defender already on the PC to avoid a conflict. Simply leave Defender turned off and OneCare will monitor for spyware. If you wish to learn more there is a post at the top of the Anti-Spyware forum that explains it more completely.
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1105575&SiteID=2
I'm moving your post to the Anti-Spyware forum, since it's not an off topic question.
OneCareBear