May we also swear by saints or any other creatures?

No; for a lawful oath is calling upon God, as the only one who knows the heart, that he will bear witness to the truth, and punish me if I swear falsely; (a) which honour is due to no creature. (b)

(a) 2 Cor.1:23 Moreover I call God for a record upon my soul, that to spare you I came not as yet unto Corinth. Rom.9:1 I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost, (b) Matt.5:34 But I say unto you, Swear not at all; neither by heaven; for it is God's throne: Matt.5:35 Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. Matt.5:36 Neither shalt thou swear by thy head, because thou canst not make one hair white or black. James 5:12 But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.